From my observations and my own personal experience, the only tank I'd feel comfortable putting a mag in would be justjoe's 20,000 gallon reef.
Short of that, a 250 gallon 4'x4'x2' tankwould probably accomodate the max size these guys can grow. Make sure there is no possible way for it to be sucked into a powerhead or CL intake and make sure your tank is both established (1 year or more) and that your water quality is unbelievable. Also, seriously understock your tank.
Keep in mind that this animal should outlive your great grandchildren. If you die before it does I'd consider you a great success.