Well, I'm glad to hear that you survived the experiment without doing any harm! Steel balls certainly seem like some tricky things to play with. I would leave it to the professionals.
I recently saw a video on the Everything Butt site in which Roxy Raye inserts one that looks like a 2.5" or 3" diameter. If you have never heard of her, she is a virtuoso if there ever was one when it comes to anal play. She has so much muscle control that she appears to keep the heavy ball just inside her ass. Or perhaps the sheer weight of the object prevents it from going in too far.
Anyway, it's quite a feat and not something I would recommend trying at home without a great deal of training. And of course whatever diameter the ball is, it would have to be gradually worked up to.
I have read somewhere that you can start out using a steel ball with a string tied through it and eventually you can get used to controlling how deep it goes to the point where you supposedly don't need the string anymore. But it is a controversial topic. Some say you should never insert anything without a way to pull it back out. On the other hand even an object like that with a string attached can go too deep and cause the damage described in the post at the link I posted in my previous comment. So you never know.
All in all, I can't recommend this sort of thing. Why not just use a comfortably sized plug and eliminate all these unknown variables, and then if that gets boring, maybe work your way up to a slightly larger plug?
By the way, here is a link to the video I mentioned:
http://www.kinkondemand.com/kod/shoot/35451-Outrageous-Anal.html
~ HC