slightly more scientifically..........
What revs are you pulling at WOT? As I'm sure you know the ideal set up is to pull the manufacturers stated max revs at WOT with your "normal" load. IIRC 4 strokes dont rev as much a 2 strokes so say 5000rpm? If you are over reving then you need to increase the pitch. If you are not achieving RPM then you need to reduce.
I would imagine with a 15incher you'll be off the rev counter! Not sure whether you'd spin a 19incher but I'dve thought a 17 inch for sure. I seem to recall that we ran with a 15 inch on the 50hp Yamaha (the second engine!) on Spirit during RB4.