Years ago I was looking for a set of anal toys that the both of us could use. We anticipated the fun of putting them in and leaving them in while we fooled around. I stopped by at a sex shop and, of course, there were a million anal toys. Some clearly too big for her desire and some that were just too weird. I talked to the clerk who strongly suggested that any anal toy we wanted to buy and that we intended to use "hands free" should have a tapered base (as mentioned above) or a flat bottom. She said that there was always a chance that even a medium sized smooth toy could go all the way in and maybe not easily come out. We ended up using two smallish toys meant for prostate stimulation. Both had a tapered base. So when we inserted them into each other, the muscle at the opening of the anus just closed around that narrow part. And that felt good too. Looked great when I slid it into her bottom. Those worked well. Over the years we tried a lot more and they were all good in different ways. Now, we both prefer longer ones as opposed to fatter toys. It seems the length feels better up there when you're changing positions or moving around like giving oral or fucking. Sometimes I'll put a very thick one in there if I intend to give oral as well as fingering her because I like feeling the toy against the inner wall of her pussy. But generally, the long ones feel good as I'm moveing around. It's like always reminding me that I'm stuffed back there.