7 inches bottoms out in me. When I was a freshman, I briefly used a diaphragm for birth control when I was having some problems with the pill. I had used it with a couple of guys, who could tell it was in me. I had just met a new guy, and the first time we had sex, I wore it, and he came out of it bruised I felt so bad. I measured him and he was about 7 1/2 inches. I guess either the other two guys werent that big (I think they were), but the new bf really seemed to know how to fuck like a pro - he seemed to really get in deeper than the rest! I told my doctor about him and bruising, and she congratulated me for finding someone who knew how to "get it done" (her words). She gave me a new scrip for the pill, which worked a lot better and more conveniently. My boobs even got a little bigger for a while.
Back to the point - for me, it seems anyone bigger than 7 inches is probably going to hit my cervix. Probably explains why the occasional "large guys" I knew in college were uncomfortable once in a while.