Well, I believe a man's size is all in his head too. But I bet I'm thinking about a different head than you, Sweety
Ilovemtl,
I truly think it's a combination of things: both enviromental and genetic.
Genetics is obvious. People's size, hair color, eye color, disability, etc. are very much determined by genetics. Penis size, at least in my eyes, is no different. However, many of these factors (such as a disability) is also combined with an enviromental factor.
Enviromental factors might play a role in the penis too. Here's an example:
When I was younger, in my teen puberty years, I was definitely a late bloomer. Perhaps this is the foundation for my desire of a bigger penis?
Anyways, I, like many men my age, masturbated quite often. It was the normal thing to do. I truly believe that this affected my size. Here's why: I went on a family vacation for about two weeks... and as a result my little guy couldn't come out and play. After I got back, he grew quite a bit. I believe that by not masturbating, and thus keeping my ever so important testosterone (and other chemicals) built up, I caused growth. Who knows how much I would have grown if I didn't masturbate nearly as much when I was in my puberty years.
Point taken: I think it's a combination of both.
Determination of Penis Size 
It will surely build over time, especialy if you push it. 
Linear Mode
