I'm thinking of going with a wider rear tire for my snow contraption. I'm currently on 1.75" knobbies, and given the clearance I'm seeing, I could probably get away with 2.25" but I'd probably go with 2" wide tires just in case the wheel goes a little out of true for some reason.

I didn't know if 1.75" was the max, though, without going to a wider rim. 2" should be fine.