Yeah that sounds strange as well. Usually a .60 sized engine would be swinging something in the 12-13 inch range. Also, if they are suggesting a 4 stroke, that will usually turn something larger too. I believe the 4s' make power lower int he RPM range, much like BL motors would. Do you have the plane built yet? What kind of ground clearance do you see from it, when it is sitting level on the bench?
Good luck,
Curtis