Whenever a creature with powerful build is subject to a size modifier for an opposed check, the creature is treated as one size larger if doing so is advantageous to him. He is also considered to be one size larger when determining whether an enemy’s special attacks based on size can affect him.
A creature with powerful build can use weapons designed for a creature one size larger without penalty, and the creature’s unarmed strike deals damage as if he were one size larger. However, his space and reach remain those of a creature of his actual size. The benefits of this ability stack with the effects of abilities, powers, and spells that change the subject’s size category.