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Well I think he might be looking for the races that allow you to wield oversized weapons (Minotaur, etc.). I don't think he is actually increasing or decreasing in size, if so I think it must be a house rule.
EDIT: oh and the reason was I'm play testing that player made 4e Druid thats floating around the boards allows Tiny<-->Small<--Med-->Large<-->Huge need to know how the size change would affect dmg
thx
Although also relevant if you wanted to change a monster size as a DM and need t adjust their natural attack accordingly