well then a ten inch diameter will do, however the pitch can go as high as you want, I believe the OS 40 will swing a 9X10 or a 10X10.... Just in case you don't know, quick study, the pitch number is the second number and represents the distance traveled in one revolution... in reality this doesn't wash sense air is not perfect.. Imagine a block of soft butter ( for a wierd way of explaining it) In one revolution the 9X6 will travel 6 inches through it, the 9X10 will travel 10 inches through it in one revolution...... if there was one, a 2X10 would travel ten inches as well, however now we factor in air.... Hmmm a slopy medium for the application for sure.... now then a 9X10 technically will not go as far in air as a 10X10 since there is one extra inch of prop to bite air........ now factor in the engine, will your OS40 swing the 10 inch prop or the 9 inch prop at full power to full potential RPM, or will the extra energy needed to swing that extra inch of prop reduce the rpm and thustly reduce the efficiency of your 10 pitch prop....... gee there is a lot of stuff to think about isn't there???????? Hmmmmmm hope this all helps....and when you are done with the block of butter, you can go get toast.................... Captain Crash...