coolbean

Use the one suggested by the motor manufacturer. Since this is a powered glider you'll want a 9X4.7 folding prop.

Edit: I couldn't find any 9x4.7 folding props, the closest I could find is a 9x5 APC folding prop. Since the motor states that it can handle a 10X4.7 slowfly prop, it should be OK. You should be trying to limit your full throttle time with that setup anyway because the battery won't take full throttle for more than 30 seconds at a time. Again as this is a sailplane I would imagine you are going to be using the motor to get up to height, try to thermal around as long as you can, repeat.