Depending on what size fox you want to catch, would mean what size trap you would want. Typicly for smaller fozxes, you want a trap with a diameter of at least 1. 5 feet. medium, 2. 5 feet.
First I'd like to clarify that it is called a foot hold trap, not a leg hold trap. The trap grips the toes of the fox, not the leg. An appropriate size trap for a fox would be a 1 1/2 coil-spring trap.