For me it's usually around 2" max diameter, but I have a number of smaller toys that I still like as well. I do have bigger toys, but they're more of an effort to use so they don't come out as often.
I got the A-Bomb a couple of years ago, thinking that if I could use a 1-3/4-2" diameter toy comfortably then a 2-1/4" toy would be an easily accomplished step. I was wrong. It took a while to get it in period, and longer to get it in comfortably. Now I can use it without pain, but it does take an hour or so to get it in.