In general the two most common APC props are the 20.5x14, and 21x14. However its really personal choice, some props like the 20x15 give lots of speed, 22x12 lots of pull but less speed, etc. etc. A lot has to depend on the sequence you fly, how you like to fly and so on and so forth. Each person has a different feel

There are also other props, PT carbon props, RASA single blade (20x15, 21x13) and RASA folding blades, Xoar makes some wood props for F3A electric.

I think you will find that the 21x14 is the common choice because it offers a good combination of pull, speed and does it quite efficiently. Its not by chance that this particular size of prop has become the most popular

Hope that helps somewhat.