From the D&D Main FAQ
Does the powerful build racial trait allow a character to take advantage of feats for size Large or larger creatures, such as Awesome Blow? Would a half-dragon goliath qualify as Large for determining whether it has wings?
No and no. The powerful build racial trait (found in the goliath entry in Races of Stone and the half-giant entry in Expanded Psionics Handbook) spells out exactly when the character is treated as one size larger than normal:
1) Whenever subjected to a size modifier or special size modifier for an opposed check. This includes grapple checks, bull rush attempts, and trip attempts.
2) When determining whether a creature’s special attacks based on size can affect him. This includes improved grab and swallow whole.
3) When determining what size of weapons the character
can use.
That’s it. The character isn’t treated as one size larger when determining space, reach, Hide check penalty, or for the purpose of qualifying for anything that requires size. Some feats, prestige classes, or other game elements may specifically allow a Medium character with powerful build to qualify as if he were Large. Such examples are specific and intentional exceptions to the normal rule.
