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
Using a reach weapon while mounted
If you're willing to go outside the Player's Handbook for clarification, a reach weapon does work a little differently for a rider atop a mount: a reach weapon becomes a little more versatile.
The Player's Handbook only says, "For simplicity, assume that you share your mount's space during [mounted] combat" (157). The handful of times creatures otherwise share spaces the creatures are foes (like during a grapple), the creatures are all really little (like a gang of grigs) or one creature's very big and the others littler (like an ogre and a gang of grigs), or one creature's helpless (like when one's dead). Because those all have clear rules and this case doesn't, precisely what the Player's Handbook actually means when it says a rider and mount share the mount's space is up to the DM. With its silence, the Player's Handbook implies that—much like if the rider's mount were wielding the reach weapon instead—a rider wielding a reach weapon is burdened with the reach weapon's lack of close-combat functionality. And, as you may've noticed, this kind of sucks.
Now, let me preface this next part by saying that the Rules of the Game columns by Skip Williams—a coauthor of Dungeons and Dragons, Third Edition—take some heat because the columns sometimes present wrong information. (For instance, this column had to be reposted because of errors.) Yet in this case, instead of creating a new rule (which the columns sometimes do), the column clarifies the above ambiguous one. The Rules of the Game Web column "All about Mounts (Part One)" says
(Emphases mine.) So, while I expect most folks (like me!) imagined a typical reach-weapon-wielding rider threatening the 10 ft. around the typical mount—and leaving that pesky unthreatened usually-5-ft. gap between the reach weapon's threatened squares and the mount so that the mount must defend itself—, this ruling allows a reach-weapon-wielding rider to, essentially, threaten through the mount so that he can also launch attacks against adjacent foes. (While the first emphasized statement's important, the second seals it: a creature can't make attacks of opportunity against foes with relative cover, and if, for example, a rider measures from the farthest-away-from-the-foe square of his mount through the mount, the Player's Handbook's unclear if the mount's granting the foe cover because rider and mount share the mount's space; this ruling clears up that the mount doesn't grant the foe cover from the rider.) St. George approves.
Keep in mind there's still an area into which a typical reach-weapon-wielding rider atop a typical mount can't make attacks: within his and his mount's shared space! The Dungeon Master's Guide says that "a creature can attack into its own space (unless armed with a reach weapon)" (29). (Emphasis mine.) Beware gangs of grig highwaymen! (But only because they're so adorable.)
Let me make clear that I, too, don't like this ruling's provenance, but mounted combat is so vague in the Player's Handbook that relying on an outside source—even a Rules of the Game column—to make heads or tails of it seems better than giving up and playing Smash Bros.
The other questions
That was long and complicated. These two'll be quick. I swear.
"How much damage does a lance deal?"
A typical creature wielding a lance one-handed deals the lance's damage plus the creature's Strength modifier. A typical creature wielding a lance two-handed deals the lance's damage plus 1½× the creature's Strength bonus (also see here). This amount doesn't change if the typical creature's riding a typical mount, although usually a creature can't wield a lance one-handed when not riding a mount (see below).
"Can a rider wield a lance two-handed?"
Sure. While a lance is usually a two-handed weapon as per Table 7–5: Weapons (PH 116-7), the Player's Handbook also says, "While mounted, you can wield a lance with one hand" (118). Thus, a mounted rider can use a lance either one-handed or two-handed like St. George up there (although, really, St. George may be using a longspear, which is two-handed, mounted or not). A lance gains—not loses—versatility when a rider's mounted.