As a femme, I have always had my lovers take it as a very high compliment when I agree to cut my nails for them. If you have rough calluses, cuts, or abrasions, just use a surgical glove. You should be, anyhow, for safer sex reasons, but even if you're with a fluid-bonded partner, grab a glove if there's anything that could scrape the tender insides of your partner's pussy. That said, there's no way to tell whether or not a given woman will be able to be fisted, or will enjoy being fisted, just by looking at her or by having other types of sex with her. Just as the overall size of a man is not a reliable indicator of his penis size, the overall size of a woman is no gauge of her vaginal capacity.
Petite women are just as likely as bigger women to be able to be fisted comfortably and pleasurably.
At most, all that having a baby will do for a woman's ability to be fisted is to convince her that yes, it is indeed possible for something larger than two fingers or your average penis to fit in there. Hand size is actually a more relevant concern. Like pussies and dicks, not all hands are created equal, and a woman who is able to be fisted to delirious moans of joy by someone with a small, slim hand might simply be unable to accommodate someone who has a big, thick, heavy-boned paw. If it hurts, you're doing it wrong, going too fast, not using enough lube, trying to fist someone whose body isn't ready or able to be fisted, or some combination of the above. About the most discomfort the fistee should feel when being fisted is a little bit of a stretchy sensation that some people find pleasant and other people find very mildly painful, kind of the same sort of sensation you feel when you're stretching out and you find a muscle that is slightly tense.