I love anal, and even at my size, some gfs or guys couldnt take it. Others loved it. I think it helps being average sized, but as others have said, if a woman wants anal, she will find a way to make it work. I'm a lover of rimming and getting a tight ass relaxed and excited, so that might help! Plus lube, i don't need a lot, but it's the anal lover's best friend.
The only times I'm fairly certain my small size was a factor is a few times with a few gfs, we'd be fucking vaginally and she'd want it in the ass. Going gently and slowly, i was able to enter right then....no lube, no rimming, or prep, just lots of natural lubrication and lust! Very hot!