Austin gave you the basic rule of thumb but I don't do any of the math. There are also several charts you can look at but the instructions give you a basic size idea of the props that the engine works well with. You pick out the basic starting prop then with a fist full of different sizes and pitches you go out and start testing props to see what one you the pilot likes best.
I like a slower plane with more pull, that equates to shorter take off runs and slower landings so I like a longer prop with less pitch.
If you do a search you can locate a photo John Buckner once posted of his prop wall, he has a better selection then most big hobby shops that he uses for testing.
In my case with your plane engine combo I would probably end up with a 12X 6 or 4 pitch. Maybe even a 13X 6 or 4?? I wouldn't know for sure until I spent the day testing props what I like best.
I have had enough .61s that I know I would be starting with a 12, probably an 8 pitch but that's just me and I know what I'm looking to achieve in the end.