Actually.... ;) Not sure how good the science is on these (haven't analyzed them), but here are a few:

A study involving taking casts of vaginas:

https://pubmed.ncbi.nlm.nih.gov/10895030/

A study comparing birth canals by population:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6234894/

Also, I can't find the studies that these ideas come from, but I remember reading somewhere years ago that vaginal dimensions might correlate with the average size of an infant's head in a given population and that the population that has smaller-headed babies has narrower vaginas. (Makes sense, since through most of human evolution, if your vagina isn't large enough for your baby to emerge, you aren't going to pass those too-small genes on to a new generation...) Moreover, I also read that there is supposedly some correlation between hip width and vaginal width, as well as torso length and vaginal length. I can't find the scientific sources on those today, unfortunately, but if you can locate them, it could be interesting. :)