We men worry way too much about length while women are more interested in width, but even more interested in how you move it. Depending on how much you spread her legs, she will feel you as longer or wider. But if you're just jackhammering away like an idiot, without stimulating her clitoris, your size won't matter for her enjoyment.
However, for the sake of your self-confidence, it might be beneficial to see how big your penis is on her. Try laying your penis on her face so your testicles touch her chin and the tip of your penis is on her forehead, maybe even a little bit beyond. Seeing that might make you realize that your penis size is quite good. Also, you probably have larger hands than your woman. Try having her jack you off sometime. That can be another confidence-booster.
If you're still feeling insecure after all that, make a life-size replica of your penis and try to put that in your mouth.