All these responses and no true answer. There is no size to give a girl a vaginal orgasm. Its not maybe 6 or at least 5-5.5 girth. Find a good lover and an open minded girl that is into it and she will have an orgasm with almost any size. I’m sure I can give my current girlfriend an orgasm without any clitoral stimulation if my penis was 4.5x 4.5.
Connect with the girl mentally and emotionally and be a skilled lover and that will increase your chances of giving her a vaginal orgasm, not enlarging your dick by half an inch. Seems like we tend to waste time on the less important things to achieve a certain goal. We all want to make our penis’ bigger and should do it for ourselves or help our insecurities, but if your main goal is to give her an orgasm by increasing the size your taking the long way around to reach a finish line that may not exist.
So to answer your question there is no certain size, or even ballpark of size. Of course a micro dick say 2x3 may run into a problem, but any average penis on 99% of the male population is capable of giving a girl a vaginal orgasm. And if you don’t work in the clitoris too and eventually you’ll get A orgasm, and that is mainly what the girl cares about. I can guarantee you all these terms you care more about than her obviously.
To quote a wise man in a movie from the 90’s……”Gold jacket, green jacket? Who gives a shit?” Vaginal orgasm, clitoral orgasm……..give her an orgasm and then work on making it better. Don’t abandon the orgasm just to get her a “different type” of orgasm because an orgasm is an orgasm. Make em feel like they have the best body in the world and they are above the rest and the only thing that matters then and if you only give her a clitoral orgasm, it will probably be better in her head than any vaginal orgasm that she has had in the past with someone just focusing on that and that only.
Enjoy sex, don’t overthink it.