Hi, Welcome to healthcaremagic community forum..
Size of the penis are affected by various factors, importantly are the genes and hormones. Well most men who are developmentally normal have a size well enough to satisfy their partner sexually, provided they know the art. Hence, when people say, size doesn't matter, it really holds good. So don't worry about your size. Be confident, you should be doing well.
But if you are still wanting to increase the size, the most effective are by surgical means. A cosmetic surgeon can help you with this. I am not sure, if any medical drugs help, unless you have an hormone deficiency