10x6 = 600
11x5 = 605
9x8 = 648
10x7 = 700
11x6 = 726
9x9 = 729
10x8 = 800
etc

First thing you do is try a prop in the recommended range and see how it pulls the airplane.
If the engine is screaming, but not pulling well, try a prop that has a power rating that greater than the screaming, underachiever. etc etc