The motors instructions usually give you a number of props that they have tested and found work well for what the motor was designed for. They usuall skip a number of pitches, the second number. That way you can try the props they state plus the ones inbetween what they show. A longer prop with a smaller number pitch will give you a slower plane with more torque like 3-D pilots. A smaller prop with a bigger number pitch will give you a faster plane with less torque, like a lot of racers and some pattern pilots use. The trick is for you to find the prop in these sizes you like best. I ike a slower plane with more thruse so I tend to go with longer props and the lower pitch. Just for grins you can look at the APC props in Towers catalog, they now list props with names like Sport-3-D-Pattern-and IMAC. If you read that you will get a better idea of what I'm talking about. Just look at the different pitches and lengths. They also have electric props listed so you can get an idea. Read the motors instructions then buy a couple of differnt sizes to test and see what you like.