If you have the space I would just go for an 8'x4'x2' as price would not be much different than a 6'x2'x2'. You can still do the same size viewing window, (if trying to cut cost) same thickness but the fish will have more space. Do a 2x4 frame (skeleton) and line it with 3/4 plywood. Makes for a very strong tank. If you build it right you don't even need cross bracing over the top of the tank. There have been several build like this. I am doing a similar build but 6'x3'x2' due to space restraints.