My 1984 Trek 720 is a 25” frame with 700c and Schwalbe Marathon Plus 32c tires. There is at least a 1/4” clearance between the tires and any frame component. I would say the limit is 35c with no fenders. I could remove the dropout adjust screws and slam the axel but then the limiting clearance would be the brake bridges (I guess that’s what they’re called or maybe the fender mount bridges).

The 720 in my size seems a little noodley but for making miles on pavement it feels real cushy. I suspect lots of frame flex if I’m wrenching on rough terrain.