Pitch is the theoretical distance that the propeller travels without slippage, in one turn. Just like you turn a bolt in a nut, one turn. By changing the pitch your engine need to work harder or lighter, so with the same amount of throttle the engine will turn more RPM with a lighter propeller (lower pitch size) and vice versa.
Pitch has nothing to do with diameter. Your pitchsize is on the propeller, mostly it is the second number that you will find on the propeller: example 14x19, then the pitch is 19 inch (diameter is 14 inch). Sometimes the number starts with the number of blades: 3x14x19 (3 blades, 14 inch diameter, 19 inch pitch)
Check your maximum RPM, drive your boat wide open throttle, full trim out (if your engine has trim) while you still have grip in the water. Check this with the maximum RPM range that the manufacturer of your engine recommends.
Example: Tim has a Campion Chase 580 with an Evinrude 150 E-tec.He runs his boat wide open throttle, full trim and gets 4.600 RPM. Evinrude describe that this engine need to run WOT 4.850 to 5.850 RPM, so this engine should run more RPM, and the engine is not giving full power right now, also this engine is not economical and efficient enough, top speed and pulling power are far below it could be.
Go calculating to an RPM that is within the recommended range. And calculate to that specific RPM (goal). Go anywhere between the lower and upper limit within the maximum RPM range. Sometimes the manufacturer tells you any additional information, like " keep your engine in the upper limit op the max. RPM range" or "Optimum engine range". But you could also give yourself a RPM goal, if you have no idea about the performance of the boat; have the goal in the middle of the Max RPM range. If you know your boat is avarage in combination with the engine; have the goal in the middle of the max RPM range. If you know your engine is very high powered for the boat; have the goal just above the lower limit of the max RPM range. If you know your engine is undersized for the boat; have the goal just below the upper limit of the max RPM range.
To judge about your boat-engine combination, keep the kind of using of the boat in mind. If you have plenty of power for boat, but you primary use is waterski-ing, you probably do not have plenty of power, but maybe just enough power, of mayby is your boat underpowered for waterski-ing activities. Same for lots of load, or taking lots of passengers on board.
Example: Tim's Evinrude has a max RPM range of 4.850 to 5.850 RPM, however Evinrude have also an "optimum RPM range" which is 5.500 to 5.600 RPM. Tim's engine has to increase about 1.050 RPM.
Until this step the size of the current propeller wasn't important, however, now is it important to see what propeller the engine currently have. Every inch change in propeller pitch size stands for a RPM change of 150 to 200 RPM. An inch lower in pitch stands for an increase of 150 to 200 RPM and vice versa. So make your calculation.
Example: Tim's Evinrude need to increase about 1.050 RPM, 1.050/175=6 Tim need a propeller with a pitch size that is about 6 inches lower. Tim was using an Evinrude Raker 26 propeller (pitch size 26 inch), now he need to try a 20 inch propeller.
All RPM's and propeller sizes are estimates, it is always: "try and error".
Step 4 and later
Once you have determined the correct pitch size, you can start lokking for and/or experimenting with different styles of propellers, but still keeping the maximum recommended RPM range in mind! Some propellers with the same pitch size give much different max. RPM, so choose in that particular style a larger or smaller propeller, once again, to keep the engine running WOT within the maximum RPM range that the manufacturer tells you!