Okay, I'm currently pregnant, so that makes me a lot stretchier than usual, and I also don't know what circumference in inches would be excessive for me, but this is what I can offer about girth limitations, in my experience.
For me, this one is hard to answer since it changes depending on circumstances. With me, I can take more width when I stretch out and if I remain stretched out due to toy usage or lots of sex. Without regular sex or dildo or vibrator use, I get smaller again, which can be a problem when TheBF goes out of town, so I use dildos and sex toys to remain stretched out for his return. If I don't, I get sore too fast and we can't have sex for long before I want to stop due to the discomfort. TheBF has told me that he can feel me getting tighter after only a few days, but he's never mentioned the tightness to me during sex (as in, "Wow, you're tight!") before two weeks apart without using any insertion toys. It's not so fun. That burning sensation from being stretched out is only tolerable to a degree. Past a certain level of intensity, it just hurts and makes me sore very fast, but a day or two later, the same girth won't have the same effect on me any more because my vagina has stretched out to accommodate him. I assume that at least some other women must experience the same phenomenon.
We've been attempting fisting. So far, 3 fingers is uncomfortable, 4 fingers is extremely uncomfortable. With four fingers, I feel like my tender parts are being crushed between his knuckles and my pelvic bones. I get a strong urge for him to just stop with both 3 and 4 fingers. He's only successfully gotten four fingers in once and he said that it was so tight that it hurt his hand, like when someone shakes your hand too hard and he's crushing your fingers.