l have a 20' pontoon boat with a 75 Johnson Stinger and was wandering what would be the recomended size prop for maxium performance on this motor?

You want to select a prop that will let the engine make 5000-5500 RPM at full throttle with a normal load. A higher pitched prop will lug the engine, less pitch will overrev it.

Decreasing the pitch by 1 inch will increase RPMs by about 200 (props of similar design). A good prop guy can customize a prop to optimize a boat/motor combination but u probably do not want to get too fancy.

That motor came with a 19'' pitch prop. If it is turning 4800 on ur 'toon, I'd probably try a 15.

On a 20 toon use a 17 pitch> 15 is a bit flat blade and to much RPM> Unless your carrying a load like using it for a barge>Then 15 will be better>