VAGINA

Down  from  the  clitoris  and  urethra  is  the  entrance  to  the  vagina.  The  walls  of  the  vagina  rest  against  one  another, creating a potential space rather than an actual one. The walls of the vagina have many folds, which explains the vagina’s ability  to  accommodate  almost  any  size  penis,  not  to  mention  a  baby  during  childbirth.  This  ability  to  expand  and contract, even to fit snugly around a finger, is the reason differences in penis size are not usually a problem.

At rest, the back wall of the vagina is about three inches long and the front wall is about two and a half inches long. As a woman  is  aroused,  her  vagina  widens  and  lengthens.  The  inner  two-thirds  of  the  vagina  balloons  out,  which  often reduces the stimulation in the back, but the outer third actually tightens as it becomes engorged, which is one reason a woman can hold on to even a small penis. As we mentioned in the last chapter, the stronger your partner's PC muscles, the more she can contract her vagina around your penis and increase stimulation for both of you. If your partner enjoys deep penetration and you have difficulty stimulating the back of her vagina, especially once it has expanded, you will be pleased to hear that there are positions that shorten her vagina and make deep thrusting easier (see the section called “Positions for Pleasure and Health” in the next chapter).
Many women report that they are most sensitive near the opening of the vagina, but other women report sensitivity at other places throughout the vagina, including the back walls and even the cervix (see the following section, "The G Spot and Other Sensitive Spots”). There are no universals about sexual stimulation, even with the best efforts of researchers like Masters and Johnson to find them. So explore with your partner, and let her tell you what feels best.