I've gotten my hand in up to and including most of my palm, but I haven't quite gotten my whole fist in yet. I don't think we're far off though. Someone with smaller hands (like a woman (she's pretty bisexual)) could do it with no trouble.
Keep working on the fisting. I do that routinely with one of my lovers and we both absolutely love it. I'm sure you've read all about it, but my advice is:
1) a VERY relaxed atmosphere, hot bath, candles, wine
2) Lots of lube (we like Astroglide) and warm in up first (sit the tube in hot water for a while). Use so much lube it runs all over the place (put towels on the bed first to soak up the excess)
3) Then go very very slowly but don't let her come. Tease her and get her needing that orgasm very badly. If you are lucky she will get to a point where she almost magically "opens up".
4) Try a lot of different positions. The position that worked first for us was a fisting version of doggie style, but now we can do it anyway we want.
5) Definitely try the "4 and 1" method also. Get all 4 fingers in her pussy and your thumb in her ass like a mitten. Then work the rest of your palm into her pussy. (Obviously DO NOT pull your thumb out and try to put it in her pussy after that!) This is easier than true fisting but will make her feel just as full. It can also produce incredible orgasms.
If she has a pussy which stays stretched easily then you can get her crazy loose this way. My lover is so stretched now that her husband is almost never able to come in her (she finishes him with her hand most times). Makes her feel wonderfully naughty when he tries and tries but can't orgasm. BUT, as you might have seen in some other threads, not all women stay stretched easily, so she might be one of those women who "bounce back" quickly. In that case she may never get all that loose, or may not stay loose very long unless you keep "practicing"