for an ABC type motor you want to use the same diameter & pitch prop recommended for flying.
also, DO NOT run it "SLOBBERY RICH" like the older steel sleve cast iron piston motors. dont let it over heat either but, slightly rich is ok.
reason is ; the way overly rich mixture will cool the cylinder sleeve while the piston gets the heat of cumbustion. being of different materials , they expand differently & it is possible to break the rod due to the piston becoming too tight and straining the rod. Its not hyper-critical but dont over do it. you may find it easier to start using an electric starter on ABC .