What is the minimum size pole to use for a Fisher set ?

You can use a pretty thin tree for the set. Back when the state of Maine allowed trappers to actually trap I used a lot of different sizes. Years ago I went on the line of a real good fisher trapper to write an article. He had several trees that were not a lot more than a couple inches diameter at shoulder height. I had never used trees that small before going with him but after that I found fisher will climb something that size no problem. Those critters climb like a squirrel.
Good luck and have fun.

The minimum size pole for a fisher set is one which you can nail a box to or three nails to attach the trap to. 4 inches or so is generally OK.

Oh, the good ol' days! Nothing better than finding that perfect tree in the perfect location with nothing but a 220, a chunk of bait and a bottle of lure in hand.
Crazy when you think about how the face of trapping has changed in the state of Maine the last decade. Kudos to those states that have stood strong and kept them!
As far as size goes I always looked for something they couldn't run up because of either size, angle or both more than a certain diameter. Good luck and enjoy!

Pole sets are not legal in PA as far as i know. Im not sure the regs are clear on that as most knw PA game laws can be intentionally vague and confusing. While going through our CR class i asked the warden about pole sets. He didn't even hesitate. The reason is of perception and that is it. They are afraid of the antis. They dont want a dangling animal showing up in the media.

Make a cubby on the ground why force the fisher to climb a pole.

that used to be an option too and News Paper boxes with 120 or 5x5. 160 and 220 had to be 45ft off the ground. That's all Gone unless you use an animal (Lynx) exclusion device

I have caught some fisher In dirt holes sets and I know lots of other guys In WI that have done the same.
45 feet off the ground? I know what you meant to type. LOL

LOL 4 ft off the ground. You bring up a good point. If you don't mind the daily checks, you're hard pressed to beat the foot hold and they have no problems with exposed pans.