Selecting the right diameter and pitch of a propeller are huge. But to do that you have to know what the diameter and pitch are and how the motor reacts to them. First of all, lets discuss what diameter and pitch are. Diameter is the the complete distance across the propeller at the blade tips. So if the prop was spinning, the circle that is made by the tips of the blades would be the diameter. According to Mercury Marine the diameter is determined primarily by the rpm at which the propeller will be turning and the amount of power that will be delivered to the propeller. The Pitch of a prop is the distance a prop will move through the water in a single revolution. So say you have a 15 pitch prop, it will move forward through the water 15 inches in on revolution.
With propellers, the lower the pitch you are running the more hole shot you will have at a standstill. But having all of that hole shot power will sacrafice your top speed. Running a lower pitch prop makes the engine get to its max range at a slower speed than it would if you had a higher pitch on there. In turn a higher pitch will give you better top speed performance but slower acceleration. Also a lower horsepower engine can bog down if propped too high which may cause wear on the internal parts.
When talking about pitch and how it directly affect the motors rpm's, just remember that lowering the pitch will increase the rpm's and raising the pitch will lower your rpm's. Each inch of pitch is about 200rpms, so say you have a 15 pitch on your boat but need to bump up the motor 400 rpm's to get into that manufacturer recommended range at wide open, you would go down to a 13 pitch prop.
It really comes down to what kind of performance are you looking to get out of your motor. If you load your boat down with a lot of gear or people, you may want a lower pitched prop. If you're looking for that top end, you may want a higher pitch. In the end, you have to make that decision and its either one or the other.