Fisting is a fairly minority taste in the swinging scene and it's certainly in the category of acts where you should ask first.
If fisting is something you're planning on exploring I would strongly recommend that you read A Hand in the Bush: the Fine Art of Vaginal Fisting by Deborah Addington who is an enthusiastic fister and fistee. Even for non-fisters this is a wonderful coffee table book.
I've included Lesbian Fisting Picnic as one of the videos for this section (see sidebar), not because I've watched it but just because it has such a wonderful title.