The video posted earlier is from the infamous KirkJ, aka Goatse Man. He obviously overstretched and I suppose he must be having leakage issues, but he is in the very very rare & extreme case.
The largest I can take is a good size fist and that's about how large I will get.
Yes, it goes back to semi-normal, but the more you do it, the bigger you can take.
After a while, say a month, it tightens up a bit. I was in the military for 2 years and of course no anal play there, only when I was on R&R or sent home for a week. Well it tightened up quite a bit, but not to the point where it was virginal again :)
I never experienced anal leakage but after play I sit with a towel between my butt cheeks just the time for it to get back to normal.