Kevin was a injected with a growth potion thought to cure Kevin from the radiation from Herb's machine that he went into. But instead is allowed Kevin to grow whenever he wanted, this was supposed to be a prank by Bobert, but this was useful when he fought Berserk Kiryu.
How big is Giant Kevin?
Does he have any powers when he's large?
No, though one power is this size allows Kevin to be immune to EVERYTHING. Not even PX-41 can infect him.