When selecting props you need to look at the diameter and the pitch. (The pitch is the theoretical distance traveled with each revolution: ex. a 14x4 prop has a 14" diameter, and will traves 4" with each revolution)

Smaller diameter with higher pitch gives you more speed, and Larger diameter with smaller pitch gives you more thrust with less speed (great for 3-d).

For 3-d flying you usually want a prop with a large diameter, and a small pitch. So look at your engine's recommended prop range, and pick the higest diameter, and the lowset pitch. For my SAITO FA-80, I am using an APC 14x4W (wide blade). There are a lot of articles on this subject if you have any more questions.

I hope this helps. --Dan