The pitch is the angle the prop blade sits to the air stream. The greater the pitch the more agressive the prop.

Normally if you go up in diameter you go down in pitch.

The engine can only do so much work until its over loaded. A greater pitch prop of the same diameter will turn at lower RPM's due to the work load on the engine.

The idea is to have your engine operating in its optimal RPM range with a prop that provides the best performance for your aircraft.

Oh, one more thing.. where's your pilot?