16" diameter will work fine if your plane weighs 10 pounds or less. My guess is you will want more thrust though, as you are at what 13+ pounds?


From manual:
"Suggested Props: Aerobatic: 15x12-14, 16x10-14, 16.5x10-13 Sport: 17x10-13, 18x10-12"

This motor likes to be loaded. Some are running a 20 x6 prop for thrust. What are you running for a muffler & what prop & rpm's are you getting now?