RE: Got my LT-40 completed
Unless you want a hot-rod, go with a 4" or 5" pitch prop probably between 10 1/2" and 11" in diameter. Diameter the engine can turn without overheating, misbehaving, crappy idle/etc will be significantly affected by variables like blade width and general blade design.
ex. a 9x8 pylon racing prop loads an engine much less than a Rev-Up 9x8 "W" wide blade.