Smeg, been there, done that... fisting is like any other sexual act, except that it requires a little more patience. The size of a fist is nowhere near the size of a baby going in the other direction.
That been said, most fisting occurs in one of a few ways:
1 - the 4 finger fist, where the thumb (and therefore the widest part of the hand) is not inserted.
2 - The petite fist: Often seen in lesbian videos, the fister has fairly small hands, and the receiver is larger.
3 - the post mommy fisting: As many of you family guys know, women tend to be "looser" after giving birth. In some cases, the girls can easily open up a fair bit, allowing for fisting action.
4 - the masochists: These are girls that are fiarly petite, but love the pain and suffering that comes from the big stretch.
For the most part, you will rarely see fisting that includes an actual FIST, which puffs up the hand, but rather what is better called a "whole hand insertion", with the hand still relatively flat (or open palmed) which isn't quite as large at the opening.
it should be noted too than in many places in the US (Including Los Angelese) fisting is cosidered an obscene act, and as such, rarely appears in US commercial porn videos.
MATt