I have fisted my husband and I have been fisted anally and vaginally (not simultaneously) by women with small hands, but never by my husband because his hands are too large.
In general I would say that with "normal" sexual practices (I use that term VERY loosely here because fisting isn't normal), you won't overstretch the anus and with practice, it will be easier to do large insertions, and it will return to its normal condition after a rest period.
You will damage the anal ring if you overdo it, and I think if you stretch it like this:
It will most likely lose its ability to perform its usual functions of maintaining fecal continence and you will end up needing anal/rectal reconstructive surgery.
Practice, practice, practice, but listen to the little voice in your head, and don't go overboard.