I haven't done anal with my wife, but recently I started to experiment using her dildo on myself. I have sometimes wondered how it felt to be on the receiving side of the penetration. I should add that the whole idea of trying what I did was completely repulsive to me in years past. At work I made no pretense to the fact that I would never have anal sex. With that as a background it wasn't like I jumped at this with great expectations or anything. Once I decided to try it, it took about a week to get it in fully, then about another week to reach the point were I can do anything I want with it even though it is 6" of girth. Two things I learned should the time ever come for my wife to insert it in me (or a guy who is about to enter his wife/gf anally) is this: assuming you or her went to the john and cleaned up prior, the receiver should use a dildo on themselves in private to really loosen things up, and if there is any feces still in their rectum it will most likely all come out when the dildo is removed. Anal sex following that is very straight forward, and the chances of things staying clean when done is greatly enhanced. Once I first get the dildo in (it takes perhaps two or three minutes from a cold start), I can do whatever I want from that point on.
A partner worrying about how her partner is going to enter her will not have any problems if she first puts her dildo inside herself in private, taking all of the time she needs. He will almost easily be able to enter her after that. If your partner is thinking about trying it for the first time, teaching herself how to do it using a dildo will make it so much easier for her the first time she has her man enter her. She will learn what hurts, how to stop the hurting, what things feel like as it enters her deeper and deeper, etc. She will have a very sound idea of what is going to happen even before and as it does, which will greatly reduce many if not all of her fears or anxieties. You now add the intimacy of the two of you and it takes it to a whole new level.