My Best advice is to use a tacho and test different props on the ground.
See how big you can go in diameter and pitch before you start to lose rpm. This should give you a good idea as to what will work well in the air.
Obviously High rpm doesn't always relate to high speed. without a tacho to measure rpm you'll be flying blind as it were.
For instance I have A Great planes P51 mustang with a Saito 65...... a 13 x6 inch prop gives considerably more rpm than the 13 x 8 which I fly it on. But the larger pitch pulls the aircraft around far better. The interesting thing is to the human ear you cant tell the difference in rpm between the to props...Good luck