I like to think of them like gears on a bike. The higher the pitch, the bigger the gear. Boat will go faster at lower RPMs to a point.
Mercruiser has a sizing chart online that will tell you what the proper size props you need based on your engine size, leg gear ratio (most are 2:1) and weight of boat.
CCB, you really only need new props if yours are damaged, too big or too small for your boat. You test the size of the prop by seeing whether your boat achieves the recommended RPM range at wide open throttle. If RPMs are too low, you go with a smaller pitch (for the Bravo III) and if too high, go with a larger pitch. When your engine is in the recommended RPM range, fuel economy will generally be the best. Smaller prop pitch will also help with hole shot getting up on plane.