okay...I'll chime in here (or hear...depending)
fisting (what it won't do): It won't make her vagina stay in the shape of your fist forevermore. The vagina is this weird elastic thingy in pops back to normal size (eventually...after at least a half hour) after having something really big in it (besides a baby's head...then I think it takes like a few weeks). Now some women may not have the strongest vaginal muscles in the world to begin with, so that fist makes a bigger difference. But if you're worried about the sloppiness factor. She could try Kegel's those are really good for keeping things nice and tight.
Fisting (what you don't want to do): Try to get your fist in all in one night. This is an excellent way to actually do some real damage. The best way is to do it over a period of time, slowly pushing the limit each time. You know, one finger, two fingers, three...try spreading those fingers out a bit. If she shrieks in pain or tries to back away...dude, you're going way to fast.
I have personal experience with the going to fast routine. I was out of commission for two weeks with a vaginal tear about half an inch in length. Of course some of this was admittedly my fault, because I'm a bit of masochist. So I would do the whole, "wait...okay, let's try it again..." even though a better response would have been "wait...okay let's stop for today"
However, my vagina is no more looser than when it started. And afterwards I did have a closer experience with the whole thing...half the hand and no pain...I was also insanely horny and extremely lubricated so that helped a lot.
