Hi Garry,
I have a 2" ball on a 3/8" shaft that's one of my favorite toys. I too like big things in my ass and, as you've found out, it feels great. I can't tell you where the limit for you will be, since each person is different.
I've had larger toys in me and have been fisted by my ex many times. I've found that for the really big objects, I have to be really relaxed and "in the mood". Some days it'll fit and some days it won't. Never force it.
One thing about a ball is that it's a bit more difficult to insert because of the lack of taper (which is what makes a ball such an intense sensation). I'd suggest that if you're looking to go to a larger object, you get maybe a 2.5" diameter butt plug. You'll probably find that the taper will make it easier to insert. Once you've achieved a larger plug, you'll find it easier to fit a larger ball.
Always remember that there's no such thing as too much lube. Also, as you stretch your ass, you'll probably experience a bit of discomfort. That's normal, just like working out. But there should be no real pain. If there is, STOP! That's your body telling you ENOUGH!
I've been doing this for a very long time and haven't done any damage. Meaning that my sphincter still closes as advertised and I don't have skidmarks in my drawers. After a session with the toys, I always do the anal version of Kegel exercises and clench my sphincter a hundred times or so. It keeps the muscles in good tone.
I've found that one of the best ways to accomodate a new large toy is to insert it as far as it will comfortably go and then just maintain a steady pressure on it. As you hold that pressure, consciously will your ass to relax and you'll feel the toy begin to slip further in to you without pain. Pretty soon, you'll have acclimated to it and you'll have achieved another plateau. Also, if you can find a partner to work with you, it makes it easier many times.
Good luck to you, and enjoy.
ldb