Think some people are getting the cervix and the back wall confused. Guys with 5 inch cocks can hit the cervix
Indeed - at a normal state, the cervix is only 3.5-4 inches in, on average. When fully aroused, the cervix moves up and lengthens the vagina to about 6". I am not entirely sure what I hit (fornix?), but I know it is a wall some 5.5 to 6" deep.
Some women claim their vaginas are 8-9" deep, but from what I have read, and my experience, I would put it more like 6", on average. I am sure there are rare women with even 10" deep vaginas, and I have heard there is a major increase during childbirth. Now, of course, different women have different sensitivities. For those who are not hurt by it, of course, the guy could thrust in more than the depth itself. Unfortunately, for me, personally, my girlfriend's end is very sensitive and too painful.