I have done quite a bit of fisting. I have found that IF a girl's had children, it's mostly possible, with care. The woman's vagina grows when she's pregnant, and the baby's head stretches it further. If a girl's not had children, that's another issue completely. When a woman enjoys it, the experience is apparently (I'm the fistER, not the fistEE) almost beyond description. As Susie says, LUBE!!! Go slowly!! Very slowly. Communicate. And don't force
things. The purpose is submission and pleasure, not harm. Obviously, there's going to be some pain as the body accommodates a much larger object than it's used to, but serious pain is not acceptable. The fistEE must be able to relax, and not panic, and real sexual arousal helps here. A LOT.
I am here discussing vaginal fisting ONLY. Anorectal insertions of really large objects may be possible, but I am not persuaded that it can be done safely. Better to limit anal play to smaller things, I think. The anus and rectum should not be stretched to the point of "no return."
I have been uniformly unsuccessful with women who've not had children. I know I said that, but it bears repeating. I have not tried "over time," in the sense that one might practice at some length.
To the best of my knowledge, I have never harmed a partner in any way, but I am VERY careful.