I know firsthand that females can get at least 11"...and males can get over 13". And that's grown out in a tank, not a big pond. I think they're still growing too.
My pair are in a 156g, (5'L x 2'W x 25"T). It's plenty of room for now. I'm lucky my pair seem to have a very strong bond.
Can you think of or have any theories as to why your pair has bonded so well? Did they pair naturally? What size were they again when introduced? You have for the most part had tank mates right?
Pics?