A 22-8 is actually a pretty good prop size for a 3.2, depending on what type of flying a person wants to do. Excellent for 3D work. For more speed, a 20" prop with a higher pitch would be better.

A reasonably good 3.2 should be able to turn an efficient 22-8 in the 6,900 to 7,400 rpm range at sea level. Perhaps more depending on the type of prop.

Above 22inch diameters on a 50cc engine is placing a lot of load on the engine, creating a lot of residual heat at the top of the piston, possibly contributing to detonation and later damage. Some will argue that they successfully run 24" props all the time, but the melt down just hasn't happened yet. The front bearings usually go first, though.