I know I had a 12in JD myself about 5 years ago but that jack was already 10 years old when I got him at that size. Got him for about $20 so by far a much better deal lol. They do get that big but very rarely. Mine was just lucky that his previous owner kept him in a 500g pond tank as the only fish with a couple turtles. I honestly think that's the only reason he got as big as he did. Wish I had a pic of him still but that was on my old camcorder that I lost somewhere in the house.
I got a 10 inch for 20 bucks as since my girlfriend liked him and felt bad he was in a 120gal at the LFS. He's old and most definitely not growing anymore but he has to be hand fed and I think is nearing his final years based on the hunchback.