No stupid question on this forum, only folks hoping to learn. I've never smoked a brisket that big, but wish I have had the opportunity. I don't change the wood weight when I smoke 10 lb or 15 lb briskets...so I would just use what I always use. I like about 3ozs of oak,2 ozs cherry, and a little hickory...but like any type of Que'n, it is a personal taste as far as wood is concerned.
Short answer...do what you normally do and you'll be happy.