Hi!
It all depends on what plane you have got!
A low winged sport plane a 11x6 (APC or RAM) is the prefered prop.
In a large slow flying high winged plane like a CUB a 12x4 is better.
For a Q-500 racer (Weight 1,6-1,8kg and 130 cm in span) a 9x7 APC prop is the way to go!