A #1 1/2 is perfect for fox. I have caught them in #2's and it would break their leg. If they break a leg they are more likely to get away, but they don't seem to fight a trap near like a coon will.
Dirt hole sets work well for fox. I highly recommend getting some fox urine to put around the trap (not on the trap itself). I have caught red fox, using gray fox urine so that doesn't seem to matter. I have even caught reds in the same trap 2 nights in a row.
