ORIGINAL: Missileman
I think it depends on the airplane too.
I found a 14x7 2 blade gave me the best performance with my OS 91FS
which would equate to a 13X7 3 blade.
all this on a 1/4 scale RV-4
Now BigTim states best performance with a 14X7 3 blade prop on warbirds.
I have been a fan of Graupner props since I flew one last summer.
They have a 12.5X7 that might be a good match for your engine.
http://www.hobby-lobby.com/grprop.htm
This is a USA distributor, the Graupner props are made in Germany here is their website:
http://www.graupner.de/156.0.html?L=1
its just the size I felt went well with the particular plane not necessaly the best,probably the best for a 91 would be the 13/7 but with warbirds I try to get the largest dia. and best performance I can,for the looks as well.
I have quite a few duds in the hangar
that really didn't fit what I was trying to do at all, its about matching the right prop with the plane,and what type of flying your doing the Mustang has a 14/6 and it flys just as well as the P-40, the OS 91 in that plane spins it up a little higher rpm's the main difference is the P-40 gets off the ground really fast and I only need about 1/2 throttle to fly at a scale like speed.