Tire clearance question

Plenty of room, I’d guess 32c. Take it to your local bike shop and have them swap in a few different wheels with different tires to get a better feel, then buy your tires through them.

You can run whatever fits in between the chain stays and the closed brakes(during breaking) you have quite a bit of room…a more important factor in making this decision is what your rim will accept, depends on the brand design of the rim. I have 2.5” hookworm tires on clincher rims that are barely 25c width but they are designed to accommodate this set ups, 32c is even a stretch for some old school design 25c rims