I've never seen any 12" (which I think is the size of a dinner plate) but I did have 1 that was 9" and 1 that was 8". They were that size when I got them, I ended up giving them back to the same shop I got them from, they were just too spastic and wild.
I don't even know the proper latin name for the 'real' Red Hooks. Most people (me previously) assume that Red Hooks are just the ones with the red hooks. But some of the common Silver Dollars have red hooks, too, so it gets confusing. I don't much care for them and just use them for clean-up duty.
ThatPetPlace says they get to 16" (
http://www.thatpetplace.com/pet/prod/213737/i/7/product.web), I'd have to see it to believe it though.