what I have found is a 14/7 3blade is a good size for a 91 4stroke for warbirds,I can get 9,200rpm without bogging the engine down, when I went higher in pitch such as a 14/9 the engine was bogging down with the extra drag,if your looking for performance then a 13/7 is a good choice just a little less drag but still getting the bite if the 7pitch prop since your engine isn't a high reving one being a 4stroke it can handle the higher pitch props.
both of these planes have OS91 surpass engines in them