I'm reading alot of the comments on here, and I'd like to start off by saying that Fisting, is often very much wrongly judged and misunderstood. It is NOT a violent act, and couples should build up to it, over time. People often assume that the woman is being penetrated with a closed fist. This is rare, and only for very experienced couples, because it can lead to damage and prolapse. You don't want that.
Now, fisting is successful, and very pleasurable because the woman experiences the deep penetration and width of your hand, why manipulating her g-spot. Penetration is done by having your hand in a "duck bill" shape---your thumb and four fingers are touching. You can start off by fingering her---and using toys to build her up to being able to accomidate your hand. Start with 2 fingers, move on to 4. When this is successful--the "duck bill" can easily come into play, and the fun begins.
The tissue in the vagina is like a rubber band, it can stretch, and recoil--and regain shape easily. Fisting WILL NOT cause the woman to have loose lips and a saggy pussy. Period----this is a myth. If you believe this, you've been watching too many porno's with women being brutalized all day, every day with no recovery time.
To the OP: as far as fisting works after childbirth---your correct, in a sense. She will never be as small as she was before delivery, but she can get very very close. Due to this, fisting may come easier. As long as your waiting at least a couple months after she's given birth, if not it is very dangerous--if its before 6 weeks after delivery, fisting can cause hemmorage, uterine prolapse, or death. So please, play safely!
Now, for those who are against fisting, but want something different--- you should look into figging, I love it... see if your woman would agree? :x