It would be my impression (owning an air keel and 5hp 2s) that 5 would be a waste of weight and gas on a roll up. You will never get it on the plane and just make more noise trying to go faster, which you won't I would say 3.5 and save weight, you'll go just about as fast. If the dinghy bottom is clean and I am alone with no cargo I can "just" plane. So go 6+ or go small.
My .02
Doug
Namaste II Falmouth, ME