20" wing span is rather small...and a Pee Wee .020 would be plenty of power if built in the half lb. range...unless maybe you're talking about a delta or sumpthin?

If you're thinking about an .049 to .074 then you must be designing a racer type airplane...[X(]

Have you checked out the 1/2A to 1/8A forums yet Panzer?
http://www.rcuniverse.com/forum/forumid_70/tt.htm