Good lord, 12 inches? I suspect this was written by someone who has not had a very large cock up their ass. Unless that was the point of the story, like:

Snape: *pulls out gigantic, unnatural schlong*
Harry: GET THAT THING AWAY FROM ME! *screams, runs away*

I also do not tend to mention size, but I suppose I tend to think in the 6-8 range. Though maybe Snape does get 9, because you're right, he deserves it.