I wish there were an exact “science of measurement” that would answer your question definitively. The vagina is typically elastic--especially when we’re younger--and will stretch to accommodate any (or nearly any) size required, but there can be male/female matches that are outside of that range.
As we get older, our vaginas become less distensible and less elastic. The tissue itself becomes less elastic as we lose estrogen, and we lose the “pleating” we had when we were younger (I’ve used the analogy of going from a pleated skirt to a pencil skirt). Dilators work by gently and gradually stretching the vaginal walls, making them open enough (called patency) to allow for comfortable intercourse.
Because of the variations in tissue elasticity, atrophy, a woman’s anatomy, and her partner’s anatomy, the goals are comfort and pleasure, not a specific dimension. Dilators come in sets of graduated sizes, so a user can move from one to the next-larger as she gains comfort with each. Some women will progress through the entire range of sizes; others will be satisfied before that.
We offer a variety of dilators, because women’s preferences vary. Our most popular, the Amielle kit, includes five sizes and a removable handle that provides more length for maneuvering. For those who prefer a solid dilator, we offer a six-inch-long option in a set of five or a set of seven, again depending on need. And for some women, the texture of silicone and its ability to be warmed makes the Sinclair Institute set of five their preference.
I hope this is helpful! I’ve very happy to hear that you’re still tending to your sexual health.