Well, lesbian here and I have noticed that with every women it is different in what size and length a place in her yoni that she likes. you really just have to get to know her and what she likes.. sometimes it is a series of steps to get her to orgasm.. for me it's definately not length because I organsm the best with my gfriends two fingers swirling around circles and her fingers are not long. hey men need to learn how to use their fingers inside .... they feel good.
I agree with previous posters about chemisty, trust, and warming up first with alot of foreplay. Foreplay helps arousal and also can help a women know that you think she's a sexy and beautiful being, and tell her that too.