I, personally, feel that an adult should be kept in nothing smaller than a 240gal. tank. My juvie was a pushover but as she was getting bigger, I could see the attitude coming out. Mine died, unexpectedly. I was home for lunch, one day and fed the tank and she was fine...came home a few hours later, after work and she was dead.
Check this link... and you'll find the story of an almost adult Queen, that a guy I know owns. She lives in a 125gal, by herself (but will be moved to a 240gal).
She's killed adult Puffers, Lions, and gutted a 16" Miniatus Grouper.
The owner said when she had tank mates, he'd have to throw in a whole, frozen squid to keep her busy, while he tried to feed the other tank mates, at the other end of the tank.
How big do they get in an aquarium (assuming large enough tank, good parameters and husbandry, proper diet, etc), surely not the 2 ft. beast I saw off Cozumel? I'm sorry your queen died like that - real bummer when they go when they seem healthy.
I checked out the guy's thread- beautiful fish and about what I expect from what I've seen and read. I think he's got the right idea keeping her in a species only and going to the 240.
Just out of curiosity, I wonder how a queen would do long term if added at the same time with a similiarly sized aggressive such as a clown trigger in a very large tank?