The problem probably has more to do with your technique rather than yyour size.
Has she had orgasms by herself or with other bfs?
If she has never had an orgasm (or infrequent ones), then you may have to be patient and persistent with your oral and finger skills. And for now, forget about trying to make her cum while fucking her.
Plan a session when it's only about her pleasure. No nookie for you-- sorry. Also, I have found that a lot of mental foreplay works well-- things you can say or do during the day long before you see her and before having sex. That stuff really puts women "in the mood".
If she is used to orgasms, then you have to let her direct the action. Listen to her! Pay attention to her body language. Does she moan or shiver when you tongue or finger her a certain way? Usually its just a very slight or subtle motion. When you notice what it is, stick to it. It may take a while. Your tongue and wrist will start to ache, but don't give up. No pain, no gain!
Hopefully, she's vocal enough so that you can hear her mounting to an orgasm.
Don't worry. Once you learn how to give her a first-rate orgasm, you'll get all the nookie you can possibly handle. She'll be throwing her pussy on your dick 24/7.