Houserule sanity check please?

[ILM]

Houserule: reach is determined not by the character's size, but by the size of his weapons.

So for my players I'll have a nice table available but since tables are a headache here, basically this means that a medium character with a medium sword threatens to 5 ft as normal, and a large character with a large (i.e. appropriate-size) sword threatens 10 ft. However, a medium character with a large sword (and associated penalties as normal, or Monkey Grip) threatens 10 ft, and a large character with a medium toothpick sword would only threaten 5 ft. I'm tempted to add in a caveat that a character always threatens at least out to his unarmed reach as well, just so there's no confusion if, say, a Huge character ends up holding a spear sized for a Tiny character.

Am I unintentionally breaking the game in two?

[ExLibrisMortis]

No, unless you're going to make it super hard to get appropriately-sized weapons, nothing will change, but it might challenge suspension of disbelief. A lot of reach comes from having longer arms, rather than longer weapons. Even amongst medium creatures, the difference between a 1.3m tall dwarf and a 2m tall half-orc is very significant (though not taken into account in the rules).

You'll have to deal with unarmed strikes, which by default have a 10-foot reach on a large creature. Apparently a large-sized ogre can hit further without a toothpick than with.

Instead of reducing reach, you could add a penalty for using too-small weapons at range. But the easiest is probably to leave the system alone. Why did you want to add this houserule?

[Ashtagon]

Personally, I prefer something that combines both RAW and what you have described.

Take the character's base reach for his size class (and shape, if using the 3.0 reach rules). Then add the following, depending on weapon size:

Small/Medium weapon (with reach): +0 (+5) feet range
Large weapon (with reach): +5 (+10) feet range
Huge weapon (with reach): +10 (+25) feet range
Gargantuan weapon (with reach): +15 (+35) feet range
Colossal weapon (with reach): +25 (+55) feet range

I haven't quite decided what I want to do about the minimum range of reach weapons though.

[ILM]

No, unless you're going to make it super hard to get appropriately-sized weapons, nothing will change, but it might challenge suspension of disbelief. A lot of reach comes from having longer arms, rather than longer weapons. Even amongst medium creatures, the difference between a 1.3m tall dwarf and a 2m tall half-orc is very significant (though not taken into account in the rules).

You'll have to deal with unarmed strikes, which by default have a 10-foot reach on a large creature. Apparently a large-sized ogre can hit further without a toothpick than with.

Instead of reducing reach, you could add a penalty for using too-small weapons at range. But the easiest is probably to leave the system alone. Why did you want to add this houserule?
Because under the current rules, a human with a Diminutive-sized whip can hit stuff at 15 feet, and that doesn't make sense; a human wielding a Huge-sized, 30-ft sword still only threatens at 5 feet, and barring deformity feats the almost only way to increase your reach is through size increases, which I personally dislike.

I agree with your comment though, and that was why I was suggesting the caveat that your reach could never be less than your unarmed reach, which I thought was a rather nifty way of sorting out the problem.

[ExLibrisMortis]

It doesn't really solve a problem though. When have your (medium?) PCs been using diminuitive whips to hit things 15 feet away, why are they even able to use such small whips, and why aren't they throwing a rock for easily 1d4 extra damage? Mind you, diminuitive whips actually have a damage of zero. I suppose they could've been tripping with them, but that isn't actually possible, by RAW.

Page 113 of the Player's Handbook has the rules for inappropriately sized weapons. I quote:

"If a weapon’s designation would be changed to something other than light, one-handed, or two-handed by this alteration, the creature can’t wield the weapon at all."

A medium creature can use a small one-handed weapon, such as a whip, as a light weapon with a -2 penalty, but not a tiny one-handed weapon, because that would become a category lighter than light. You could wield a tiny glaive as a light weapon at a -4 penalty, or a large whip as a two-handed weapon, with a -2 penalty. In the first case, you technically get 10-foot reach (although with lowered to-hit, power attack and strength modifier, making it a terrible choice), in the latter case you keep the 15-foot reach of a regular whip, trading a little attack and your off-hand for a little damage.

So: your houserule only applies when your PCs are trying to use huge light or large one-handed reach weapons (because you can't use two-handed reach weapons of a larger size at all), or very small reach weapons, such as small whips (still 15' reach, and since you were only using them for tripping anyway, the lack of damage is fine) and tiny glaives (still 10' reach). I suggest you simply ban the ability to have reach from any weapon that isn't used in two hands and be done with it.

Oh and, a huge sword is probably not 30 feet long, that's a gargantuan longspear you're thinking of. Huge swords are about as big as large two-handed swords, so about 10'.

[Fouredged Sword]

Here is something I have heard done, two handed weapons become reach weapons when you wield with as oversized weapons. It doesn't address multiple sizes over your size though.

So here is what happens at+1 size - -4 to hit,

+1 size
Light weapons -> one handed weapons
One handed weapons -> two handed weapons
Two handed weapons gain reach

Creatures with powerful build choose how they want to wield a weapon at the start of their turn, but never take the -4 penalty.

[ILM]

Here is something I have heard done, two handed weapons become reach weapons when you wield with as oversized weapons. It doesn't address multiple sizes over your size though.

So here is what happens at+1 size - -4 to hit,

+1 size
Light weapons -> one handed weapons
One handed weapons -> two handed weapons
Two handed weapons gain reach

Creatures with powerful build choose how they want to wield a weapon at the start of their turn, but never take the -4 penalty.
Hm, that's an interesting way of doing it. I like that, thanks!