sudas: I can't remember my source for the height comment. I just remember the AH-HA feeling, when I read it. My mind's been in the gutter for so long, I don't know on what street it is. I think I'll sumarize what's been discussed:
1) Arousal- consensus says it increases depth. There's a post around here where the girlfriend's one was deep all the time. Previously, I thought that was the norm.
2) Greater height from shoulders to bottom- more likely to have larger organs? This is my contention; certainly they have more skin.
3) Distance of belly-button to orifice? More data needed, please. I saw one girl with a lot of skin showing below her belly-button, wearing low pants. Perhaps this meant she has a deep one (and shaved).
4) Being able to put your fist in your mouth? Dubious relationship.
5) Practice? Maybe it works for width, but I suspect it's related to #1.