The best thing to do, in my opinion, is to buy one of a few different sizes. Spare props are always good to have, and at three or four dollars per prop, it's not an overly excessive cost. Also keep a tachometer on hand. Put the prop on that you would like to try, tune the engine for optimum operating RPM with that prop, then put it in the air and see if it meets your requirements.

Remember not to over tach any engine. Manufacturers usually put an RPM operating range but often, the high end is a tad bit optimistic. Operating that size engine at 15,000 RPM is somewhat excessive (again, in my opinion) and will make a lot of noise and will probably not give you what you want. I try to maximize my operating range to a maximum of 12,000 RPM on smaller engines and 10,000 on larger ones, then prop it accordingly, varying pitch to make my requirements for climb, speed, and pulling authority. And, I don't usually fly my planes at wide open throttle for long, usually for take off then when I want to go vertical for some maneuver. Other than that, it's usually at half or so throttle. I get good performance and fuel economy operating it like that, and try to choose props and tune to meet that.

Everyone is different, though, and I am not inviting an arguement here, just saying what I do to optimize my engine/prop combination to how I like to fly my planes.

DS.