Perhaps the penis is a muscle that grows like any other. How do you train it?
It is a sponge. If you could implement some protocol during puberty such as hGH, L3IGF-1, and DHT cream, all applied locally, perhaps you could grow a horse's cock. But after a certain age many things no longer grow so I do not think you can make it grow appreciably larger as an adult, with GH or anything else. At least not practically speaking.