Usually (not always) the diameter and picth is stamped somewhere on the prop. Either on the outside of the body between the blades, or inside it, on the support braces.
Hard to choose a prop without knowing how the motor spins. RPM is key; speed is sort of irrelevant (unless you've got the rpm right and are looking for a tad more fast.) If the current prop is usable, take a car tach/dwell tuneup meter, and go for a ride to get rpm with the throttle wide open. That'll give you enough to rough guess what kind of pitch you'll need (assuming you find out what you've got now.)