A lot of men find it hard to believe that women to not care about the size of a man's penis. Yet, women know that there are men with sizable erections, who are terrible in bed, but there are others with small ones, who turn out to be great lovers still. Then what matter to them?
What women need the most is communication. It might be weird at first, but you can talk about sex, and what both of you would want in bed. For instance, when a woman sees a big penis, she will ultimately say no to hardcore blowjobs, and anal sex, as well. They would also oppose to using toys in bed to simulate a double penetration. However, a man with an average or small cock can get this without problem, if they tell their partner what they want.
Also, do not ever think that once you have taken off your clothes, they will laugh and say no to sex just because of your size. They will have sex with you, because like I have mentioned, they do not care about your size. They want to feel what you can do with it. As long as you do not care about your size, neither will the girls.
If you know a woman's anatomy, you will be able to give her an orgasm anyway. If you do it in doggy position or if you are on top with her ankles on your shoulders, you can penetrate her vagina deeper, and she will feel as if you had a big tool indeed. If it is still a challenge for you, then feel free to use your fingers. You can get her off that way, as well.