Every young guy seems to worry that he might not "measure" up and that some guy is bigger and better. Some really "big" guys have trouble getting "in"...it can hurt a girl.
When you are roughly 20-21 years old (sexual peak), your penis will have grown approximately as long as it will ever get, but as you get older, the girth will increase some...more at the base. You can't easily change it so don't worry about it. v1agra type medications, however will make it longer and wider just before you ejaculate, but it's NOT a permanent increase. Best idea is to accept what you have as long as it works right.
Remember, it is not the size that counts so much, but what you do with it and your sexual technique. Any girl worth dating should be more concerned about what's attached to the penis, than the penis itself. Remember to play safely.