That's a tough one to answer with what you have provided. Is there a prop on the motor right now? The diameter of the prop can not change, as is fits the prop shaft for that motor. You can only play with the pitch to adjust your rpm's and performance. What is the model of the motor? The ideal pitch for your prop would be based on what you are getting for RPM's at WOT with the prop compared to your engine's specs on its allowable WOT rpm's. For each inch you change the pitch up or down, you will lose or gain about 200 rpm's, depending on if the pitch increases or decreases. You need to know the motor's recommended max RPM's, then see what the prop you have is giving you for rpm's on the pontoon at WOT. Then you can go up or down in pitch to get as close to the max RPM's as you would like. I know this might not be much help, but without knowing more, its hard to guess what pitch will work for you set up. Cheers!