drywaller
by top end do you mean air speed or engine rpm? these type planes like air speed thus more pitch is good. it doesnt hurt to load the motor some. I mistakenly put a 10 x 5 ( 5 looked like an 8) on a 40 sized chipmunk the motor sounded great but did not want to hold level flite well. also depends on how lite the plane but for a 60 size 11x 7.5 dont sound too bad a place to start