In diameter, I would have to say I put an entire 10 oz enema bulb syringe in me. (DO NOT TRY THIS AT HOME!!!!!) Or at work, or school, or the beach, or Walmart, or anywhere elce for that matter. I know I never will do that again. I had just taken a 2quart enema, and I thought I would be clever and use the bulb syringe as an inflatable butt plug. I just squeezed the syringe as flat as I could, folded it in half, lubed it up, and shoved it in. I did this in the shower so it wouldn't make a mess as I knew I would loose a few ounces putting it in. I put it in open end first so the pressure in my colin would fill it up and plug me up. Up to that point, everything worked perfect. When time came to un-cork, that's when the nightmare began. I actually had to push it further inside me to relieve some of the watter pressure first. I was wondering if this was going to end up with an embarrasing trip to the ER. But after getting psyched enough to push the thing out, I went for it and got it out on about the eighth or tenth try. Yes, I was bleeding a little bit afterward. Yes I was in a lot of pain afterward. Yes I stayed away from enemas and anal play for a long time afterward. And no I didn't have to see a doctor afterward; I just got extremely lucky. That was several years ago. And today, I still say "DO NOT TRY THIS!!!!!".
As for length, I would have to say about a ten inch beaded anal plug, just last week. Big Deal! Woo Hoo!!