There is no precise answer to your question.
In a none aroused state vaginas are typically 3" to 5" inchs in length.
Now as for the aroused state, it varies per person but generally the maximum
depth is between 6" to 9" inchs.
It is very rare for a vagina to be stretched beyond 9" due to anatomical features within the body.
Also if you have seen a large amount of illicit movies, you will have noticed that those endowed with extreme length are never able to fully penetrate a vagina.
Thus, length is good but only to a certain point, anything beyond that is useless. (only aesthetically pleasing, not physiologically pleasing)
In any case we should all stop worrying about sizes so much and just enjoy the moment