depends where your engine makes its power. if you're spinning the 7x6 at, say, 16,000 rpm you'll need to 20,000 on the 7x5 and 24,000 on the 7x4 to get the same speed...but you will get more thrust. can the OS .15FP spin those sizes that fast? i'd bet that it runs out of poop quite a ways before 20K due to conservative porting and would stick with what works best right now. if you need more speed the go up in pitch but down in diameter, but remember that you'll be giving up thrust (vertical) and acceleration. for more of that use less pitch and larger diameter.

the best way to know what really works on your engine/plane is to go flying with a bunch of different brands and sizes of props. while doing this you also have to do your best to ignore the sound from the engine as noise can be easily mistaken for speed


dave