Weapon size relates to the size of the weapon in comparison to that of the creature wielding it. This determines which weapons can be wielded, as well as how they are wielded by a particular creature. In addition, the relative sizes of weapons are a factor during disarm attempts.
A creature can wield any weapon up to one size larger than their own size and down to two sizes smaller than their own size. Other weapons cannot be wielded. (For playable races, this only excludes large weapons from gnomes and halflings.)
A melee weapon at least one size smaller than the wielder is considered a light weapon.
Example: A medium-sized human may wield a medium-sized longsword in one hand or a large-sized greatsword two-handed. However, a small-sized halfling may not wield a greatsword and may only wield a longsword two-handed.