i think the biggest ever assembled was the one chad smith played on. sa detroit yata yun. they attempted to break the world record (or SET it). chad smith's friend owns a drum shop there and i think publicity stunt na rin for his shop.
EDIT: inassume ko lang na by "biggest" you meant the most number of drums in a single set. hehehe. as to the biggest in SIZE, i guess google might help us. hehehe.