The limitations are not always because of size, but also ( like already said by Khristy) by choices or like in my case something else.
My friends wife doesn't want him to do anal penetration with her anymore with our threesomes, not because of his size ( large or small ) but because of him cumming way to soon. She's really into anal, only after the right preparation and playing for getting her in the mood, but finds it a real cold shower having him slide in and cum right away or after a few thrusts.
After us meeting separately and she comes home, then telling him what dirty or crazy things we did, she likes him to play with her and when she's close to cumming he's allowed to penetrate her in any way he likes.
This, cumming way to soon, is also one of the reason years ago they asked me to join them.