My girlfriend achieves orgasms from both clitoral and vaginal stimulation. I believe people are referring to stimulation of the surface of the cervix and the vaginal fornix (area of the vagina just beyond the cervix). The required penis length to do this sort of stimulation is simply going to vary with the length of the woman's vagina (and whether the uterus is retroverted or anteroverted, etc). When a woman is not aroused, it may be easier to reach with the penis. As women become more aroused, their vaginas tend to lengthen a bit, making the fornix harder to reach. So, probably somewhere around 5.5 to 7.5 inches on average.