i thought they only got 12 inches but i guess i read wrong.I think a 75 or 90 would be good for a single specimen,and 125 for 2.since most 125 gallon tanks are 6 feet long.
The biggest you can afford. Its gonna be easier to put ina 6x2x2 once than to start with a 55 then move upto to a 75/100 then need to move things again to a 150. Ok so its gonna look a bit sparce to start with but youd be giving them the best start from day 1.