I'm currently working on a room 15' x 12' and went with a 600cfm exhaust.
The reason being that I can get away with using >9" duct work. The 12" would have been imposible for me to use(common with larger exhausts). You need alot of clearance for 12" ducting.
http://www.ventingdirect.com/index.c...h&keyword=HLB6