RE: What prop size for .15 Sport UC plnes?
In the end the best prop for any plane/engine combination is the one that makes the plane fly as close as possible to how you want it to go.
I tried a Peacemaker (Rustler Olly power) with an APC 10X4 last Saturday. Just loverly, may go back to an 8X5, or a 9X4 just to see how it changes the flight characteristics.
It's all a learning experience that you must do for yourself if you want to optimise the performance for your style.
G