I've never had any experiences with fisting anyone except for this once:
I was 24, picked up a skinny blond guy in a bar, probably late 20's. He worked in a medical office, we went there after the bar closed to do our deed. Spread his legs, sank my cock in balls deep...no friction whatsoever. It was like trying to screw the grand canyon, the only time my cock wasn't big enough to satisfy someone. After a few jabs and not being able to feel a thing (literally a cave beyond the slack opening), I hoisted his ass up on a table and gave him what he needed. It was intuitive and he was obviously...experienced. I only used saliva as lube and my hand slid right in. He could have given birth.
On the other end, when I was the same age my ex decided he wanted to work me over and managed to get 4 fingers in but my hole wouldn't get any bigger. His hands were average. Tried for hours and used a bunch of lube but decided it wasn't mean to be. Since then, nothing larger than a big cock.