My limit is about 2.1 inches.
Anything larger hurts quite bad. Although I have seen other go a lot further.
At first I was limited by my pelvic bone (so I thought) but with enough relaxation and going in at an angle, I was able to get objects in of larger size.
Unfortunately, I my limit is from 2 or 3 hemorrhoid I have on my inner sphincter (atleast that is what I think they are).
It sucks because I am only 19 (I think I got these from back when I was 10 way before I knew there was some better way to get stuff out through enemas), and have the desire to do so much bigger, but I cannot without experiencing a lot of pain, and possible small traces of blood I have seen in the past.
I stick to the njoy 2.0 pure plug (2 inch diam) which is my limit.
Funny enough I am so used to that now, I can put it in without any lube in the shower!
I guess what also helps with that large plug the grooves in the sides.