depends on the fields imo, small fields, small dogs catch foxes bigger dogs wouldnt even get near to put a bend into IMO. As for size, in theory it will take more stick, but in reality, a little dog cant play with a fox the way a big dog can, therefore if its to succeed it HAS to get a good hold and keep it. JMO on the dogs i have seen work.
Ideally though your looking at the wrong dog if you want a fox dog