Do you know that the vagina is 3 to 4 inches (7,5-10 cm) deep...See

 
The size of a woman's vagina varies from woman to woman (just like the size of a man's penis varies from man to man).

Unaroused, the vagina is an AVERAGE of 4 inches deep/long (think of the length of a tampon, including applicator). During arousal, the vagina lengthens and widens (just like a man's penis during arousal) ready to accommodate a penis.

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Childbirth affects vaginal muscle tone, not length unless she suffers a prolapsed uterus - so after kids she might feel wider/looser unless she does pelvic floor exercises to tone those muscles back up again.

Isn’t it funny that many women don’t know the depth, that is the length, of their vagina? When asked, they often say “15 cm. or  so”.  With that length they obviously relate to the length of a penis in erection. In fact the length of the vagina has a direct size-relation with the length of the forefinger with an average of 8 cm. How is it then possible that an erected penis can fit?

A relaxed vagina

The vagina is in fact a tube of stretchable mucous tissue, much like the mouth.  When aroused, muscles around the vagina relax . (Contrary to the muscles of the penis that contract and stiffen the erection.) So the “relaxed” vagina can take the volume of the erected penis during intercourse, also helped by the lubrication of the arousal. Also when the cervix will lift up and move out of the way. If not aroused enough the penis can hit the cervix.
This explains why intercourse can be painful when the woman is not aroused. Nevertheless intercourse with “very well endowed men” is a problem.  Also a mouth can only take a certain size of apple.

The truth about the vaginal orgasms

In general, the length of an erected penis is not what gives pleasure to the woman but the girth. This is because the sensitive tissue of the clitoris envelopes the vagina. What we see as clitoris is only the tip, comparable to the glans of the penis. So with the movement of intercourse with a rather thick penis these sensitive clitoral tissues are rubbed from the inside and this explains a vaginal orgasm.  This knowledge dates from 1998 when the true anatomy of the clitoris was finally determined! Isn’t that amazing?

The average depth of your vagina

The average depth of a vagina - it is something to wonder about. Take a guess about your own length.. how deep do you think it is? Off course is the depth for each woman different, but statistics say that on average a vagina is only 3 to 4 inches (7,5-10 cm) deep. And now think of the average length of a penis…
That doesn’t add up…  the average penis is considerably longer.  The reason is that the vagina elongates during intercourse to accommodate the entire length of the penis. When you’re sexually aroused, the upper two-thirds of the vagina will stretch out. The vagina also lubricates to help ease penetration.

A little too deep?

Then in some cases it’s still possible that a penis hits the cervix. This may be an indication that you are not aroused enough. When you’re more aroused, your vagina will elongate and the cervix will lift up and move out of the way. Other times, contact with the cervix can happen if a penis is larger than average or if the thrusting is too deep. Talking with your partner about it — "that's a little too deep" — and/or changing sexual positions could be helpful.
Another thing to keep in mind is that your vagina is not an open canal. When you’re aroused the vagina walls open apart.

Cervix

1. 
   1
   Know where your cervix is located. The cervix is the lowermost part of the uterus, where it connects with the vaginal wall. It is located 3 to 6 inches (7.6 to 15.2 cm) inside the vagina, at the end of the vaginal tunnel. It is shaped like a small donut with a tiny hole in the center. The position and texture of the cervix change throughout the ovulation cycle.

   • The inner canal of the cervix contains glands that secrete vaginal mucus. The color and texture of the mucus also change throughout the cycle.
   
   
2. 
   2
   Wash your hands with soap and warm water. Since you'll be using your fingers to feel your cervix, it's important to wash your hands thoroughly to prevent transmitting bacteria to your reproductive system. Avoid using lotion or hand cream before feeling your cervix, since the ingredients in these products can lead to vaginal infections.

   • If you have long nails, you might consider giving them a trim before feeling your cervix. A long, sharp nail could scratch your vagina.
3. 
   3
   Get in a comfortable position. Most women find that a sitting position (rather than standing or lying down) allows for easy access to the cervix with a minimum of discomfort. Sit on the edge of your bed or bathtub with your knees apart.
4. 
   4
   Insert your longest finger into your vagina. Gently move your finger into your vaginal opening and let it glide into your vagina. Depending on where you are in your ovulation cycle, your finger may reach several inches into your vagina before you feel your cervix.

   • If you wish, you may lubricate your finger with a water-based lubricant to help it glide in more easily. Do not use petroleum jelly, lotion, or any other product not specifically labeled for use in the vagina.
5. 
   5
   Feel for the cervix. The tip of your finger will touch the donut-shaped opening at the end of your vagina. You'll know it's your cervix if your finger isn't able to keep reaching further. The cervix may be soft, like pursed lips, or firm, like the tip of your nose, depending on whether or not you are ovulating.
   
   

Part 2 of 2: Knowing the Signs of Ovulation

1. 
   1
   Determine whether your cervix is low or high. If your cervix is "low," meaning just a few inches from your vaginal opening, that means you probably aren't ovulating. If it's "high," located deep within the vagina, you may be ovulating.

   • The first few times you feel your cervix, it will be difficult to determine whether it's high or low. Keep feeling every day over the course of a month or two, noting the differences in the position of your cervix from week to week. Eventually you'll be able to tell whether your cervix is low or high.
2. 
   2
   Determine whether your cervix is firm or soft. If your cervix is firm and tight, you probably aren't ovulating. If it's soft and has some give, you may be ovulating.

   • The texture of the cervix during ovulation has been described as feeling like a pair of lips. During other periods, before and after ovulation, it's more like the tip of your nose - slightly hard with less give.
3. 
   3
   Determine whether your cervix is wet. During ovulation, the cervix will feel very wet with fluids, and you'll likely have an increased amount of vaginal discharge. After ovulation, the cervix will feel drier until menstruation occurs.
4. 
   4
   Use other methods to verify whether you're ovulating. In addition to feeling your cervix, monitoring your cervical fluids and recording your basal temperature can help you figure out when you're ovulating. This combination of tracking methods is called Fertility Awareness, and done correctly, it's an effective way to determine when you are fertile.

   • Just before and during ovulation, your vaginal fluid will become heavy and slippery.
   • When ovulation occurs, your basal temperature will rise slightly. It's necessary to take your temperature using a basal thermometer every morning so that you catch the rise in temperature.