Rules Compendium p. 150 (emphasis mine):
A reach weapon is a melee weapon that allows its wielder to strike at
targets that aren’t adjacent. Most reach weapons double the wielder’s
natural reach, allowing the wielder to attack at that reach but not
within its normal reach. A typical Small or Medium wielder of such a
weapon can attack a creature 10 feet away, but not a creature in an
adjacent square. A Large wielder wielding a reach weapon of the
appropriate size can attack a creature 15 or 20 feet away, but not
adjacent creatures or creatures up to 10 feet away. Tiny or smaller
creatures gain no advantage from reach weapons.
Rules Compendium p. 151:
A wielder gains no reach from a reach weapon that is too small. No
additional reach is granted by a reach weapon that is too big.
Based on those two things, we can answer your two questions:
- No. Tiny creatures threaten no squares around them, and reach weapons don't benefit them. As a larger reach weapon doesn't convey any extra bonus, they don't get any reach no matter how big the weapon is. (Though the mental picture of a tiny creature using a gargantuan halberd is pretty funny.)
- If the weapon is inappropriately small, they do not gain a reach boost at all. If it's inappropriately large, they do not gain an extra reach boost, but they would gain the normal reach boost for a creature of their size using a reach weapon. So a Dwarf using a large spiked chain gets the same reach as a Dwarf using a medium spiked chain.
I think your proposed feat is going to work as you intend. A fine spiked chain won't convey any reach, it'd just be like using a melee range weapon only doing a lot less damage.
Specific Q/A
Basic Question: Does a grig (MM 235) wielding a Tiny longspear (PH 116, 119) (2 gp 5 sp; 0.9 lbs.) threaten no squares, adjacent squares,
or squares 10 ft. away?
The grig is tiny, using a reach weapon. It therefore threatens no squares.
Then: What about a grig wielding two-handed a Small spinning sword* (Secrets of Sarlona 137-8) (50 gp; 3 lbs.)?
Threatens no squares. The spinning sword definition says that it's a reach weapon, and tiny creatures do not gain reach from reach weapons.
Then: What about a human wielding as a light weapon a Tiny longspear?
Threatens his natural reach only. Undersized reach weapons convey no reach, but they don't take away your natural reach (since said human could just punch that square).
Then: What about a human wielding two-handed a Large spinning sword (100 gp; 6 lbs.)?
Yes, the human would threaten squares 10' away, and 5' away (as the spinning sword works at close range, like a spiked chain).
Does The Rules Compendium Contradict The SRD/DMG?
No, it doesn't. The SRD's reach definition is as follows (emphasis mine):
Glaives, guisarmes, lances, longspears, ranseurs, spiked chains, and
whips are reach weapons. A reach weapon is a melee weapon that allows
its wielder to strike at targets that aren’t adjacent to him or her.
Most reach weapons double the wielder’s natural reach, meaning that a
typical Small or Medium wielder of such a weapon can attack a creature
10 feet away, but not a creature in an adjacent square. A typical
Large character wielding a reach weapon of the appropriate size can
attack a creature 15 or 20 feet away, but not adjacent creatures or
creatures up to 10 feet away.
Tiny creatures have a natural reach of 0. If you double 0, you get 0. Thus while the RC definition is lengthier, it doesn't contract what the SRD or DMG say.
The only thing in the SRD about oversize/undersize reach weapons is the bit in the above quote about using a reach weapon of the appropriate size. The RC expands on this, but again doesn't contradict what is said. (You could argue that in the SRD, an oversize reach weapon also doesn't grant reach as it's not "appropriate", but they didn't really spell out what they meant so I don't find the RC version problematic.)
Can A Tiny Creature Ever Get Reach?
Yes, as the DMG mentions:
Tiny, Diminutive, and Fine creatures have no natural reach. They must
enter an opponent’s square (and thus be subject to an attack of
opportunity) in order to attack that opponent in melee unless they are
armed with weapons that give them at least 5 feet of reach
That makes it clear that tiny creatures are not forbidden from getting reach, if they have some way to get it. Some examples of ways to do that:
- Shadowstrike (MIC) - On a weapon, activated to give that weapon +5' to it's reach. As it's just a flat addition to reach, it works even if the creature has 0' reach normally.
- Lunging Strike (PHBII) - Feat that allows you to make an attack with +5' to it's reach. Again this is a straight addition, so it works even if you had 0' reach.
- Inhuman Reach (LoM) - Feat that increases your character's natural reach by +5'.
What About Whips?
Whips have a different wording than the other standard reach weapons, as shown here:
The whip is treated as a melee weapon with 15-foot reach, though you
don’t threaten the area into which you can make an attack. In
addition, unlike most other weapons with reach, you can use it against
foes anywhere within your reach (including adjacent foes).
Compare to the Spiked Chain (another reach weapon that works in melee range):
A spiked chain has reach, so you can strike opponents 10 feet away
with it. In addition, unlike most other weapons with reach, it can be
used against an adjacent foe.
The "X has reach" wording on the chain is the same as you see for every reach weapon in the SRD, except the Whip. The first part of my answer covered how those work with larger and smaller creatures. But is a whip special?
- The strictest possible reading says that a whip is treated as a melee weapon. It also happens to have a 15' reach, and that number is specific. Does it change for larger or smaller creatures? The rules don't say. If you wanted to do a very strict reading, it's always 15', no matter the creature size. This starts to make no sense with gargantuan or larger creatures as it actually lowers their natural reach, and would give tiny creatures 15' reach with a whip and 0' reach with any other weapon, so it doesn't work very well at the table.
- A less-strict reading would instead say that a whip is a reach weapon (that can also strike in melee, like a spiked chain) that adds triple to the natural reach, instead of double for normal reach weapons. That means a whip scales up for larger creatures like other reach weapons do (except farther), and tiny creatures get no reach at all when using one. This has the virtue of being consistent with how reach weapons normally work. Also working in the favor of this one is that the reach weapon definition mentions whips as a reach weapon.
So for whips, it comes down to how much stock you put into the odd wording compared to other weapons. I can't give a reference for what the writers actually intended to do there, as one doesn't seem to exist. I do know that the second option is far more consistent in how it plays, so I would certainly favor it in actual play.
Best Answer
Order of the Stick only references the actual rules tangentially
First of all, that joke relies on the rules of Order of the Stick being slightly different from those in both 3.5 and Pathfinder: in the actual rules, you cannot provoke an Attack of Opportunity from a given enemy more than once per action, so after you first leave an opponent’s threatened square and provoke an Attack of Opportunity from him or her, you are free to continue moving without provoking again for that movement.
Rich Burlew has stated repeatedly that he sticks to the actual rules only when he thinks it’s funny to do so, or it advances the plot.
Dungeons & Dragons 3.5 options
Nonetheless, similar tactics do work, and in 3.5 at least, the Spiked Chain is a pretty good weapon for it (the feat cost hurts quite a lot though). The typical approach is to use a Reach weapon that can Trip (Guisarme or Spiked Chain are the typical choices), take Combat Reflexes and Improved Trip, and trip anyone who tries to move around you too much. This is one of the most effective tactics available to most mundane warriors in 3.5.
Feats
Combat Reflexes and Improved Trip, as already mentioned, are key here.
Exotic Weapon Proficiency (Spiked Chain) is a pretty good feat here since you can threaten adjacent squares, but you can also use Spiked Gauntlets, Armor Spikes, and so on with a Guisarme to accomplish similar results. Depends whether or not you have room for another feat.
The Stand Still feat is also quite good for this kind of build, and Knock-Down is also pretty good. Stand Still gives you a good option against those you cannot trip for whatever reason, and Knock-Down effectively doubles your damage against those you can. If you have Tome of Battle, Evasive Reflexes brings you quite a bit closer to how the Order of the Stick character actually works, since it allows you to move back instead of taking an Attack of Opportunity (and it doesn’t count as taking your Attack of Opportunity, which means you still can and he still provokes if he moves forward – you still are only going to get to make one AoO per movement action, though).
Races
Strength or Dexterity bonuses are good, Bonus Feats are great, and Large size is awesome but usually over-priced. I’d probably go with Human (or a Human variant that still gets the Bonus Feat, like Azurin or Silverbrow or whatever), Orc (ideally Water Orc), or a Goliath from Races of Stone for Powerful Build and the possibility of gaining true Large size from Mountain Rage (see below). Half-minotaur is very good template here, also netting you Large size, though like Goliath the LA hurts.
Note that if you do not get Improved Trip as a bonus feat (see Wolf Totem Barbarian, below), you’ll need Int 13 to get Combat Expertise. Note that if you go for a Psychic Warrior or War Mind build, you’ll want some Wisdom.
Specific classes
Barbarian 2 with the Wolf Totem is a great start for such a tripper: you get Improved Trip without taking Combat Expertise, which is great. Rage is useful since Strength covers your Trip checks, and you could switch to Ferocity to get a Dexterity bonus (read: more Attacks of Opportunity) instead of a Constitution bonus. The Lion Spirit Totem alternate class feature from Complete Champion gets you Pounce, which is a great option for any melee build (though it does not directly improve your Trip-lockdown ability).
If you are a Goliath (Races of Stone), you could instead take Mountain Rage, which makes you Large for the duration of your Rage: this is excellent for a Trip-lockdown build.
Also look into the Knockback feat, also from Races of Stone, and consider combining it with Fighter 6 using the Dungeoncrasher ACF from Dungeonscape: you’ll broaden your abilities to do both Bull Rush and Trip. So Barbarian 2/Fighter 6 is not bad for a pretty simple build that has a few options. A couple of levels of Fighter are pretty common for such a character just because you do want a lot of Fighter feats. Don’t take too many, though; after you get your core feats, additional feats frequently matter less and less and you want to get real class features.
You will also want to increase your size as much as you can. Trips involve Touch attacks, which means BAB is not super-important to you: consider levels in Psychic Warrior to get the excellent expansion, not to mention being a fantastic alternative to Fighter for Bonus Feats. The War Mind prestige class is also pretty good for this. Cleric is a great dip in general, and can also get you righteous might, a pretty good size increase. Note that you have to be careful about when you Rage if you are a Barbarian and a spellcaster or manifester.
Pathfinder options
In Pathfinder, you can still do it, but not with a Spiked Chain, and you need two feats to do what Improved Trip used to do for one, and you’ll burn through your Attacks of Opportunity twice as fast. In short, Paizo decided that one of the few functional mundane tactics in 3.5 was somehow “too good” and nerfed the hell out of it. They also decided that “just because it costs a feat to use” is not a sufficient reason for an exotic weapon to be be better than a martial one, in one of their most derided and bone-headed fiascoes to date, and thus made the Spiked Chain now pretty much useless and in no way worth a feat. It was only barely worth a feat in 3.5.
Being Large would have helped the Pathfinder Spiked Chain situation somewhat since you’d get Reach that way, but it’s still much better off with a Reach weapon than with a non-Reach weapon even when Large, and more importantly there’s still nothing about the Spiked Chain that actually makes it worthwhile. So use a Guisarme or Heavy Flail instead of a Spiked Chain, and still try to increase your size.
Pathfinder also has Stand Still; it’s slightly different (Combat Maneuver check instead of attack plus Reflex Save), but the two are probably close to equal in usefulness. I’m not sure if there are any Pathfinder feats that serve as stand-ins for Knock-Down or Evasive Reflexes, though.