I'm sure the question of a medium character using a large weapon was asked often here, but what about the reverse?
Say a medium sized character wanting to use a small dagger, like strictly as a throwing knife are there any rules for that?
creature-sizednd-3.5epathfinder-1eweapons
I'm sure the question of a medium character using a large weapon was asked often here, but what about the reverse?
Say a medium sized character wanting to use a small dagger, like strictly as a throwing knife are there any rules for that?
Weapons underwent a massive design change between D&D 3e (A&EG) and D&D 3.5e: the former has weapons divided by size (a longsword is a medium weapon, which means a huge giant uses it as if it were a dagger), while the latter has weapon sizes (a longsword is a one-handed weapon, a dagger is a light one and you can have normal longswords and huge daggers and while they do the same damage they are different).
Hence, there are no huge weapons designed for medium creatures in D&D 3.5e.
Instead of delving in the rules for wielding a weapon made for a creature of different size (a huge giant can not hold a medium longsword, as I discovered in the worst moment, but can hold a two-handed medium weapon as if it was a light weapon with a -4 penalty to hit) I'd just head to the better (but costly) alternative.
You can use the Sizing weapon enchantment (Magic Item Compendium, p. 43) to have a weapon that changes to your size when you morph with a swift action. Since you're not wearing but wielding it, I think it doesn't get subsumed in your new form.
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:
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:
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?
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
They are the same as the rules for wielding a too large weapon. It´s the same in both D&D 3.5e and Pathfinder, I'm linking the D&D 3.5e SRD below.
Link to the SRD
Example: wielding a Small dagger as a Medium creature
When a Medium creature tries to throw a Small dagger (let's assume you just killed a Goblin and took their weapon?) you're trying to use a weapon that's one size wrong for you. That means you'd be taking a -2 penalty when using.
However, the second part of the rules says that you need to change the designation of the weapon by one step. In this case, one step down because the weapon is too small. A dagger is normally Light, and there is nothing that goes down one step. That means the weapon is simply too small for you to wield, so in this case you're not allowed to use it at all.
Example: wielding a Small longsword as a Medium creature
Let's say you grab the Goblin's longsword instead. The longsword is normally a one-handed weapon. Moving the designation down one step, that means you get to use it as if it were a light weapon. Since the size category is still off by one, you take a -2 on attack rolls with it.
Example: wielding a Huge shortsword as a Medium creature
A Huge shortsword is off two categories (in the other direction). That means you're taking a -4 penalty to attack rolls with it. Moving the designation of the weapon up two places, it goes from being light to being two-handed, so you can swing this massive shortsword in two hands, awkwardly.