Liveaquaria recommends 180 gallons, but like Joe Vandal says, it is not just volume that matters, but tank dimensions. They need a nice long tank so that they can swim, without being like a prisoner pacing in a cell that is only a couple of strides long.
Back in the 90's i foolishly let a LFS tell me I could keep a baby hippo in a 30g. Over the course of the year I had it, we battled and beat ich, but lost the fight with lateral line erosion. These fish really don't do well in undersized tanks.
That is why with my current 75 g I decided to get a one spot foxface rather than a tang for my large fish. I believe the one spot stays a little smaller than the Lo.
What is the footprint of a 93g cube though? Again, you want them to have room to move. That's why I am not a big fan of cube-shaped tanks.
93G Cube is 30X30X24" Add plenty of live rock there is barely room for anything bigger than 3", now that I think about it :0