When you get older and have sex with several women, you will find out that there is no magic girth. Some women will have vaginal orgasms, but the majority will not, and it doesn’t matter if your girth is 4” or 7”.
I can vouch for that. I’m 6+ inches in girth and there’s no guaranteed orgasm. I still have to grind it out.
Don’t fret yourself out over the vaginal orgasm thing my man, especially with the younger chicks. Take Zaneblue’s great advice. Focus on the clit, but not to the extent that you’re a heat seaking “I’m gonna give you an orgasm” machine. Get into a chicks *mind*. Enjoy yourself. Explore. Drive her totally fucking nuts. Take your time. Most of all, explore very carefully the responses you get to the different things you try. I’ve been amazed at how few times a chick who likes it rough actually fits the stereotype in her demeanor, dress, and (what I thought was her) personality. I’ve had really wild looking punky chicks tell me that I was too rough (so I backed way off and played the smooth caressing touchy feely thing with them) and I’ve had soroirity sisters in polo shirts and docksiders *demand* that I choke the shit out of them and play rape fantasy games. Remember that they’re mysterious and constantly changing. Look at it as a puzzle you enjoy tinkering with and whose solution constantly demands new and exciting solutions.