Mine is 19 gauge chicken wire. I think in theory a fox could chew through it but tbh, he'd have to be completely desperate and I shut all the poultry up at night anyway. I went more for the approach that it should go high and underground. I can't remember the size of the holes but small - I'd guess 1.5 inch, maybe 1 inch. I do know that mice can get in though - I found two drowned in the duck bath one time - but I can't imagine rats can (unless they can squeeze around the door somehow).
Having said that, i'm thinking of setting up a bigger run for next year when I'll have more breeding groups and if I do, I want to get weldmesh. The problem with the chicken wire is that it's hard to get tight so it flops about a bit. I haven't costed out the weldmesh yet though - definitely a lot more expensive.
While you're planning, make sure you think about the posts too - better to get good quality treated wood while you're at it. We only have one bit that wasn't pretreated timber which was the gate and it's already showing signs of weathering and that's only a few months in.
H