kinda depends on what you want in the tank, ehh? corals vs. fish vs. mixed.
there's probably a reason why there are "standard" sizes for the 120 / 180. the 2x2 cross section splits the difference.
personally, if money was a non factor (but still staying within the realm of "reality", ie, no GIGANTIC 2000 gal system), i would want a 8' x 32" (w) x 22" (t).
i like a tank that has depth, and my short arm makes anything longer than 22" annoying to work on.