The size of a melee weapon is indicated in the weapons table. Weapons can be increased or decreased in size to make oversized or undersized versions suitable for larger or smaller creatures.
An oversized weapon is one size category larger, weighs twice the regular weapon, and does an extra 1d6 points of damage. Oversized weapons cost three times the regular price.
An undersized weapon is one size category smaller and weighs half the regular weapon.
The wielder of a weapon must use two hands to use a weapon one size category larger than himself, and must use his STRENGTH attribute for attacks even when the regular sized version offers an option to use AGILITY. Weapons two sizes larger cannot be used.