I'd actually heard decrease diameter by 2" and increase pitch by 1". Not that it really matters that much - unless you want to do some REALLY complicated math (which probably won't have any effect in the real world) it's all just guess work and "rule of thumb". Try a couple of sizes and see what you like. If you want to get into the math side of it find a copy of the ThrustHP program, punch in the range of "ideal" 2 bladed props for your motor and note the disk load numbers, then start punching in available 3 blade props and compare disk load numbers. The same load should theoretically give the same power at the same speed. (Don't pay too much attention to the max speed ratings in that software though, ThrustHP is a bit optimistic there.)