Ranged weapons are not classified as light, one-handed or two-handed, unless they are also melee weapons. Yet there are times when this classification is necessary, like Sunder, but also Two-Weapon Fighting.
Thrown weapons are classified for the purposes of Two-Weapon Fighting:
Treat a dart or shuriken as a light weapon when used in this manner, and treat a bolas, javelin, net, or sling as a one-handed weapon.
Damage rolls when using a melee weapon or a thrown weapon (including a sling). (Exceptions: Off-hand attacks receive only one-half the character’s Strength bonus, while two-handed attacks receive one and a half times the Strength bonus. A Strength penalty, but not a bonus, applies to attacks made with a bow that is not a composite bow.)
And for Composite longbow:
A composite longbow can be made with a high strength rating to take advantage of an above-average Strength score; this feature allows you to add your Strength bonus to damage, up to the maximum bonus indicated for the bow.
The wielder of a two-handed weapon on a sunder attempt gets a +4 bonus on this roll, and the wielder of a light weapon takes a –4 penalty.
These are clearly references to the category, not the hands used to wield them. For example, when wielding a one-handed weapon with two hands, you would not gain the +4 bonus.
Up to this point, the longbow is basically not classified, so the determination is up to the DM. It isn't clear cut, since while the weapon requires two hands to wield, you do not gain the one and a half times Strength bonus.
However, if we look at the Elvencraft Bow, from Races of the Wild (pp 166)
An elvencraft shortbow functions as a club when
wielded as a melee weapon. An elvencraft longbow functions
as a quarterstaff when wielded as a melee weapon.
If we use this as a guide, then a shortbow functions as a club, a one-handed weapon and a longbow functions as a quarterstaff, which is listed as a two-handed weapon. However, if we read the text on quarterstaff:
A quarterstaff is a double weapon. You can fight with it as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting with two weapons, just as if you were using a one-handed weapon and a light weapon. A creature wielding a quarterstaff in one hand can’t use it as a double weapon—only one end of the weapon can be used in any given round.
Since text trumps tables, you begin to scratch your head...
Because with two-handed weapons:
Two hands are required to use a two-handed melee weapon effectively.
Ultimately a longbow simply isn't classified in the rules-as-written as a light, one-handed or two-handed weapon, thus it's up to the DM.
This DM would recommend, with respect to two-weapon fighting:
- Treat any bow (except a hand crossbow that specifically can be fired one-handed) as a two-handed weapon
And with respect to Sunder:
- Treat longbows and heavy crossbows as two-handed weapons and shortbows and light crossbows as one-handed weapons.
But those are purely recommendations at this point. You might rule differently, in that you might think the hands
on the item are where the bonus should come from, and read it as
wielding with two hands instead of wielding a two-handed weapon.
I would not go any further than that, however, since you may introduce weapon size bizarreness (like dark wanderer's mention of dual wielding small shortbows). One could argue the two hand requirement comes from the text, however, not the size, so it is a mechanical function of the weapon itself, and not the size of the weapon (hence a shortbow still requires two hands).