fisting is interesting - like anal sex, there is a safe way of doing it and a lot of unsafe ways of doing it.
if the soreness you're feeling is the same kind of soreness after a night of hard penis-vagina intercourse, then you're body experienced the same shock/stretch.
when someone fists you, they are not making a fist with there hand. that's almost impossible unless someone is EXTREMELY LOOSE or you are causing them EXTREME PAIN. you put all your fingers together, lined up tight - then, you fold your thumb into your palm - then, keeping that position, kind of curl you fingers in together so that your pointer finger and pinky touch - all of your fingers should still be straight - almost (kinda) like the hand position you use when trying to make a shadow puppet like a duck (does that sound completely confusing?) well, anyway, if anyone ever tries to fist you without THAT SPECIFIC position, they don't know what they're doing and will probably hurt you. oh yeah - you put a giant glob of lube in the center of your palm to deposit it inside your vaginal cavity.
fingernails must be EXTREMELY SHORT - that can make you bleed. if he/she bites his/her nails, they should bite them really low (i know that's gross, but it's the truth), or cut them very very short, AND file them so they are soft and rounded.
lesbians have been doing this for ages and ages - as have a lot of other people (but this is my experience) - and no health problems have occured as a result in me, or in the lesbians/bisexuals/etc that i have talked to about it.
my only concern is that your boyfriend's hand is big - although, i'm assuming that you're speaking of a boyfriend fisting you, which may not be the case. i don't know about that - i've had plenty of hetero sex, but never a guy fisting me.
my girlfriend has a very petite hand, and her fisting me was the first time i experienced female ejaculation. it was great!
i would suggest researching it on a lesbian/queer oriented site - there's a whole lot of experience to yeild wisdom from...good luck!