I'd start with a 10x5 and experiment from there.  Are there any 'lectric 3-bladers?

Yes, you can use a gas/glow prop.   A gas glow prop is thicker, heavier, and often has wider blades than the electric prop in the closest style from the same manufacturer.  Electric is lower vibration and generally run at slower RPM than glow.  Using a glow prop on electric is one way to solve prop flutter problems.

The other way around is a bad idea though.  Starting and engine vibration can break the prop.

Phil

Thanks for the reply Tim.  Yes, they do make 3 blade props for electric but could you use a gas/glo prop?

The problem with gas/glo props is they are usually thicker and heavier.. which will use more batteries vs electric props.