have you ever flown a cap? if not, they are friggin bricks. they are incredible flyers, but they have a high wing loading and stall at high speeds. a cap of that size will weigh about 12 lbs, and the engine i am reffering to is the os ft-160. it swings up to a 20x8 prop. i don't know if that helps, but i would probably run just off of guessing, maybe a 19x6