Oversized arrows

[Captnq]

So, I take a long bow for a medium sized critter and I get an arrow for a large sized creatures. I fire the arrow using the long bow. Can I?

Do I need a long bow for a large sized creature?

Should I treat the long bow medium sized as a short bow large size??

If it can fire, is damage 1d8 or 2d6?

[ksbsnowowl]

As far as I know, the rules never actually have sizes for arrows.  You could fire a pixie arrow from a Gargantuan giant's bow.  With a minor exception for specific arrows in Races of the Wild, it is the bow that determines the damage, not the arrow.

I know I've never seen any DM put up a roadblock to a halfling rogue using the arrows filched from the bodies of human guards.

[Captnq]

Okay... so now I ask...

A weapon’s size category isn’t the same as its size as an object. Instead, a weapon’s size category is keyed to the size of the intended wielder. In general, a light weapon is an object two size categories smaller than the wielder, a one-handed weapon is an object one size category smaller than the wielder, and a two-handed weapon is an object of the same size category as the wielder.

The measure of how much effort it takes to use a weapon (whether the weapon is designated as a light, one-handed, or two-handed weapon for a particular wielder) is altered by one step for each size category of difference between the wielder’s size and the size of the creature for which the weapon was designed. 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.

And...

Longbow

You need at least two hands to use a bow, regardless of its size. A longbow is too unwieldy to use while you are mounted. If you have a penalty for low Strength, apply it to damage rolls when you use a longbow. If you have a bonus for high Strength, you can apply it to damage rolls when you use a composite longbow (see below) but not a regular longbow.

So, it's regardless of size. So, in theory can I:

1. Reduce the size class of a long bow down to small and thus make it one handed? (the answer is obvious, but it builds on the next point.)

2. If I can't, because the hand number is two hands regardless of size, is there any upward limit to how big I can make a bow? (As long as I'm willing to suck the -2 per size class, that is. A -2 to hit for base 2d6 damage might be worth it. that's a jump from 4.5 average to 7 average damage.)

[Garryl]

So, it's regardless of size. So, in theory can I:

1. Reduce the size class of a long bow down to small and thus make it one handed? (the answer is obvious, but it builds on the next point.)

No. It needs at least two hands, regardless of size.
Edit: To clarify, it would be a one-handed weapon that still needs two hands to use (see below).

2. If I can't, because the hand number is two hands regardless of size, is there any upward limit to how big I can make a bow? (As long as I'm willing to suck the -2 per size class, that is. A -2 to hit for base 2d6 damage might be worth it. that's a jump from 4.5 average to 7 average damage.)

Still can't. It doesn't say it needs exactly two hands to wield, it says at least two. Nor for that matter does it say that using two hands is sufficient to wield it regardless of size. It's just an extra condition. If you somehow made yourself treat a bow as a light weapon, you'd still need two (or more) hands ti use it.

Further, by the weapon size passages you quoted, we need to determine the new weapon's designation to determine if it's wieldable. Assuming it's a two-handed weapon when properly sized (most projectile weapons don't explicitly list their equivalent categories as such), the increased size category would change it to something other than light/1-h/2-h, and thus rendering it unwieldable. But if you could wield it (Strongarm Bracers/Monkey Grip type effects), you'd need at least two hands.

[dipolartech]

The first SRD quote up there makes it sound like something like Monkey Grip would allow you to wield a larger bow just like you could wield a larger  category 2H sword. Key being here something "like" Monkey Grip because it has a specific clause for melee weapons,

You can use melee weapons one size category larger than you are with a -2 penalty on the attack roll, but the amount of effort it takes you to use the weapon does not change. For instance, a Large longsword (a one-handed weapon for a Large creature) is considered a two-handed weapon for a Medium creature that does not have this feat. For a Medium creature that has this feat, it is still considered a one-handed weapon. You can wield a larger light weapon as a light weapon, or a larger two-handed weapon in two hands. You cannot wield a larger weapon in your off hand, and you cannot use this feat with a double weapon.

Bold mine to point out that the real effect of the Monkey Grips seems to be to give a creature that is one category smaller than what is supposed to wield it the ability to wield it using the same set up as the larger creature would.

EDIT: I'll have to go rummage a book for the exact verbiage on Strongarm but from the debates over at enworld it sounds like they dont have the melee restriction.