Here are a couple of observations, though not about Shaq.
My best friend in HS was 6' 10" and he had a disproprtionately small penis. He was maybe 4" flaccid, but did not get any bigger hard. He just jutted upward.
As far as pro sightings, a friend of mine belongs to a gym with Scottie Pippen and I have seen him in the locker room several times. He has an enormous flaccid penis. It looked huge even on a guy of his size. I would seriously guess 9" soft. My friend mentioned that he had seen Dennis Rodman in the same gym and that he was of a similar size.