Generally you use a higher-diameter, lower pitch prop for better climb and acceleration, and use a lower-diameter, higher pitch prop for more speed. The best place to start is in the range that your engine manufacturer recommends, and move on from there depending on what it is you want to do. The one investment you should make is in a variety of prop pitches/diameters that are appropriate to your engine. That'll let you feel the differences for yourself, and allow you to pick the one that best suits your style of flying.