I ended up doing this with a woman I was seeing briefly about 10 years ago. We did'nt plan on it or anything, I was just fingering her and she just got more and more into it. Pretty soon I had 3, the 4 fingers in. Then I thought "what the hell" and went for the whole hand. It fit. I didn't think it would, because there was this other girl I was seeing before her, and we came close but no go. That wasn't a planned attempt, either. Just a little clit licking/fingering action getting carried away. Athough her pussy was willing, her pelvic bones were too close together for my hand to go in. Both women were about the same size, about 5'4-5'6 110-120 lbs. Just different bone structure, I guess.
Once my hand was in, I could rotate it around and move my fingers around, but I couldn't open my hand. It was almost like having your hand in a small plastic bag, in a certain sense. I could move my hand around in different directions as long as I kept my fingers together, but I couldn't spread my fingers open because her pussy would only get so big, like a small sandwich bag on your hand is the only way I can describe it.
Thats the only time it happened. I wasn't interested in doing it again. The girl was a freak in bed, though (obviously). She even asked me to piss inside her pussy, but I didn't. Where the hell is it all going to go? Yup, all over the bed. the hell with that.