Generally speaking, every inch is a 200 RPM change. So going from a 23" to a 21" is 400 more RPM at WOT, or a 19" 800 more RPM at WOT. Going from 3 to 4 blade is worth about an inch of pitch, so a 23" 3 blade to a 19" 4 blade would be about 600 RPM higher at WOT.
Ideally try to keep the WOT RPM at the redline engine RPM. Evaluate current WOT RPM and MPH. Evaluate current RPM at your riding speed. You want to be in the meat of the engine's torque range at riding MPH to be able to hold speed easier and more consistently. Pitch low enough for an acceptable hole shot at your typical weight. once you have your numbers, compare potential props by using a prop slip calculator and RPM calculator. Personally, without knowing exactly how your boat currently performs but having previously had a 21' SBC powered I/O, I'd go with a 19" 4 blade and see how it does.
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