What is a good size engine for a CAP-232. Wingspan is 77 inches. We have put a 31CC(ryobi) weed-eater engine in the plane and it will get it off the ground but that is about it. We are thinking of putting a 46CC(stihl) weed-eater and hope that will pull it around with authority.

Any suggestions?

try a ys 1.40. they are very expensive engines but you will absolutely love them. their throttle transition is seamless, it has more power than most 1.60 engines, and (the cool part) the idle adjustment is completely seperate from the transition adjustment so if you are mounted inverted, you can sit there and lean out the low end until it idles forever and the transition will not be too lean. they are competition engines, meaning they are designed to compete furiously with all other engines (including 2 stroke) for the same weight, but while offering the power of a top end 2 stroke they have the advantages of 4 strokes as well. trust me m8, you will enjoy that little engine.