Just as there is not a strong correlation between the height of a man and the size of his penis, there's also not much a difference in the depth of a woman's vagina relative to her size. This subject was explored in a 2006 study of more than 3000 women that compared vaginal length to various surgeries, age, height, and weight. In the end, while there is a statistically measurable difference based on these factors, it's not one that's realistically noticeable.
To set our baseline, the average vagina is roughly 6 inches deep when fully aroused. Considering that the average man has a penis that's 5.5 inches long when fully aroused, it seems the two definitely did co-evolve for maximum compatibility.
We are, of course, only talking about adult women. And as with men, as women age their bodies change. But the changes are not substantial when it comes to vaginal depth — every 10 years of age reduced the total vaginal depth by 0.8 mm. That's not exactly a noticeable difference, even from 20 to 60 you're looking at less than a quarter of an inch difference.
The bigger issue that women's vaginas face with age is dropping estrogen levels leading to dryness and poor lubrication, though this is helped substantially by upping your use of foreplay and lubrication for sex. Women may also experience a looser and shrunken labia, as the skin loses elasticity and blood flow is reduced.
Two surgeries that many women have done as they get older have positive, though minor, impacts on vaginal length. Hysterectomies (removal of the uterus) on average resulted in a 0.63 cm increase in vaginal length, while pelvic reconstruction surgery that repairs the pelvic floor muscles and ligaments (typically due to weakening and injury from childbirth) led to a 0.22 cm increase in vaginal length.
Menopause, which typically occurs during a woman's 40s or 50s, happens when the woman's ovaries start producing less of the hormones that drive the menstrual cycle and arousal. The study also found that women that had gone through menopause saw an average vaginal shortening of 0.17 cm versus women of the same age group that had not.
Somewhat counterintuitively, heavier women were measured to on average have deeper vaginas. While the number was statistically significant, it's not something that you would find significant in reality: every 10 kg of weight increase correlated to a 0.2 mm increase in vaginal length.
Running out the math here, a woman that's 200 pounds would, statistically speaking, have a vagina that's a whole 1 mm deeper than a woman of the same age and height that weighs 100 pounds. Again: statistically significant, but not significant in reality. It's worth noting that a woman with more a lot more body fat will likely have more fat padding around the vaginal opening, so while her vagina itself isn't any deeper, there's more flesh to push through before getting to it, and more padding to cushion a man's thrusting. That's why some larger men have a stated preference for heavier women, not because their vaginas are any larger, but because the fat provides a buffer for a longer penis.
The common perception is that height is correlated with genital size, both amongst men and women. But that height = bigger correlation can typically only be applied to things that are bound by the size of their bones — tall people have longer limbs, but not longer intestines. Similarly, while there is a very slight height-to-penis-size correlation that is statistically significant, in reality it's not so much.
The same applies to women and vaginas: for every meter of additional height, vaginal length increased by 0.9 mm. To repeat: 1 meter in height adds less than 1 millimeter of vaginal length. It's hard enough to find two women that are 39 inches different in height, so while this was a statistically measurable difference, it's not one that bears and clinical or real world significance.
Body size has no meaningful impact on vaginal depth
So there you have it: the size of a woman's body and her age has no significant impact on her vaginal capacity. Tall or short, skinny or fat, young or old, when aroused the average woman's vagina is roughly 6 inches deep. So go for the women that appeal to you.
There's no good way to pre-determine a woman's vaginal length just by looking at her. Some tall girls can struggle with slightly above average penises while some shorter ones can take a monster cock all the way to the hilt. Maybe you can try to warn her that you're well-endowed in case you're concerned about her ability to take you, but the truth is that there's no good way to do that.
The better thing to do is to know how to handle yourself going in. Foreplay, lube, and restraint are vital before you discover just how much of your cock any particular woman can take. Plump the depths with caution, and listen to her when she tells you that's far enough.
Astroglide water-based sexual lubricant
There's zero shame in helping things slide right along, and few lubes do that better or more affordably than Astroglide.
Überlube silicone-based sexual lubricant
Silicone-based lubes like Überlube are safe for internal use and with condoms, and doesn't dry out anywhere near as quickly as water-based lubes.
Ohnut depth limiters
Dick's just too long and you keep smashing your lady's cervix? Throw a few Ohnut depth limiting rings around your base to provide a stop before you go too far.