The average length is in between 5 and 6 inches. Girth, I think is 4-5 ish inches or so.
There is no real "cure". You shouldn't think of it as a "cure". As long as you know how to use your penis to please a woman, size doesn't matter. In fact, only the front third of the vagina has orgasm causing nerves. The rest is just tissue.
There are pills (which generally don't work, so I've heard) and stretching techniques you can use. Generally, with stretching, the result is little and it takes a long time.
If you happen to be overweight, losing weight actually makes your penis look larger. It also has less 'body' to hide behind, so the actual length of your penis can increase by losing weight.