I think that just as with other biological entities, we are influenced in a large part by our genetics.
As for size being hereditary, I would think that this is true in most cases. I'm sure there are cases that fall outside of this though the reason for their occurrance would be hard to explain. What might be difficult to take account of is the part of the genes that come down through the maternal side.
In my case I am definately a "grower" not a "shower". At sixteen I went on a canoeing trip with my Dad, the summer before high school. While aware of my changes, including penis size, I wasn't really fully aware of my circumstances.
The two of us were travelling through a fairly remote area and I thought that while out in the wilderness I would spend some time working on an really "even" tan. What I was doing was skinny dipping for hygene and then laying out on the rocks or beach as we travelled. We went our separate ways for a couple of hours to get the washing up done though not so far that we would get lost. I had several sessions of tanning and sometimes would fall asleep. I knew enough to know that I surely didn't want to get a burn so I used the alarm on my watch to wake me if I happened to fall asleep. With the fresh air, light breeze and sunny days I would sometimes wake up with an erection, not a rock hard one but full sized.
About a week or so after getting home my Dad said that with the coming of the start of high school, it might be a good time to get a physical done. I figured that was okay by me and didn't think much about it. I went to the doctor's and he ran a battery of tests and took blood samples. He did the "turn your head and cough" as well and then made a second appointment to review the results. At the second appointment he went over the results and everything was fine then he explained to me that my Dad had set up the appointment because he had seen me sunbathing nude, asleep and pretty much at full size. He, my Dad, was concerned that my size might be a result of a medical condition and that he didn't want to worry me so he got me in to the doctor's in order to be checked out. I guess I looked pretty freaky to him and the doctor thought it would be a good idea to just check and be sure. This was also when I first realized that I was a bit above average. The doctor also gave me some advice and said if I had any problems to make an appointment.
Though my father and I had "the talk" prior to this and we had some discussions after; my size was never mentioned.
It was a visit to my uncle, my mother's brother, that clued me in about my mother's father. He noted that we looked a bit alike and that we both resembled my grandfather. At this point he asked if I had gotten any "nicknames". I said no, not really. He then told me that his aunt, grandfather's sister, had asked him the same question years before she had died. Apparently my grandfather was nicknamed "horse" and his aunt had made it clear it wasn't because he was strong, though he had lived on a farm with her before he went to work on the railway.
