All women are different.
Some enjoy deep penetration. If you are unable to do this yourself you might want to consider buying a vibrator that you can use on her while you stimulate other areas such as her clitoris or around her anus, whatever she's comfortable with....
Some women are able to bring themselves to orgasm with their fingers and your fingers can be just as efficient as her own! If she knows how to bring herself to orgasm, ask her to show you how and then you can take over! If she doesn't know how then that might be something that the two of you can discuss finding out!
As long as you are considerate in regards to your partners feelings you should be able to talk about it all! If you care about each other and both want the other to get the most out of your intimate experiences as possible then talk to each other. Experiment with one another and you will find what works for the two of you!
Size does NOT matter! Where there is a will there is a way!!!