A consultant urologist will give you the best surgical options, and everything else should be regarded with the utmost suspicion. In general the erect penis is a pretty constant 5 to 6 inches, and the flaccid penis can be anything from 2 to 6 inches. The penis is constructed from 3 tubes of sponge (which empty and fill with blood to give the enlargement and stiffness), one of which laps over the top, (the glans), surrounded by skin and other binding tissue. The enlarging substances go into the surround, and you cannot increase the amount of sponge, so you can see that there are bound to be problems. Make one yourself out of sponge strips and fabric and you will see what I mean.
In the heterosexual world, penis size is not generally much of an issue, except in porn movies. In the gay scene it matters a lot more.