You can take pills for it (Make sure there all natural, I'm taking extendaquin) or do exercises such as the jelqing technique which is basically just grabbing your penis and pulling all the blood to...
It really depends on genetics. It doesn't HAVE to be any size. Depends on what you want to do with it. To urinate, 1 inch is enough. To get a woman pregnant, it realistically takes 3 or more inches.