"Our ability to understand size as it relates to sexual function is poorly understood," Christopher Tarnay, director of the division of female pelvic medicine and reconstructive surgery at the UCLA Medical Center, told WebMD. "But in the last ten to 15 years, there has been a lot more attention paid to the field of sexual medicine."
The important thing to remember is that the vagina is an elastic organ, and it can go from being very small to expanding to large enough to allow a baby to pass through it. This is because the walls of the vagina contain rugae (also found in the walls of the stomach), which allow it to fold together and collapse when unused, then expand when necessary.
According to WedMD, the most commonly used measurements to look at the size of vaginas come from Masters and Johnson’s work from the 1960s, which found that vagina lengths, unstimulated, range from 2.75 inches to about 3.25 inches in women who had never been pregnant.