Dodge like Bob said that is a really loaded question and is predicated on several differant factors several of which are what kind of airframe, how you intend to fly it and what is your skill level. because you posted here on the beginners forum the answer on any one of those engines will be differant than the same question you asked on the pylon forum.

For example at the pylon forum you asked about props for the 40X on a Dominator Q500. If you are actually going compete the it would depend on the local rules and for AMA 424 prop requirements vary from area to area Examples of which would be APC 8.8 X 7.5, 8.8 X 8.5, 9 X 6 (C-2 pattern) and so on. If your intent is to sport fly that airplane then props in the ten inch range (10 X 6, 10 X 7 etc) would be more appropriate.

So you see without knowning a little more about you, the aircraft and its use there is no way to answer your question.

John