Mercury's website offers this advise:

o Adding 1 inch of propeller pitch will reduce full-throttle RPM by 150 to 200.
o Subtracting 1 inch of propeller pitch will increase full-throttle RPM by 150 to 200.
o If you're moving from a 3-blade to a 4-blade propeller, a 4-blade generally turns 50 to 100 RPM less than a 3-blade propeller with the same pitch.

My question is, "What's the paradigm for prop diameters? All other things equal, what's the RPM change for one inch increase in prop diameter?"