Pretty good advise from John, prop size has been discussed here over the years with the rule of thumb posted so many times it could fill a page all it's own. The owners Manual shows props in the working range of the engine then it's up to the pilot to test props until they discover the prop make and size they like the best. I have my new little Up-Roar I scratch built with the FOX .40 and started out with a 10X6 master airscrew. I am looking for a slow flight with as much thrust as I can get. At the moment the plane is a bit faster then I was looking for and I haven't started prop testing yet. I'm betting an 11X4 or 6 will be what I end up with. I just couldn't locate enough props in the sizes I was looking for to start testing. The MAS 10 inch props with a 6 pitch was were the only props I could find at my LHS. I found a bunch of APC props in the sizes I was looking for but they were all reverse thrust?? The LHS must have gotten some kind of deal on them??