Well, for the larger tangs (Acanthurus, Naso), size does matter. The tank would be better if it was longer as opposed to more cubish to allow for these fish to swim and really grow out well, unless you're sporting a 5' cube
For a 3.5'x2' tank, I'd stick to Ctenochaetus tangs, and maybe a zebrasoma if it's one of the smaller ones, and a smaller specimen too.
edit: I see I don't understand humor. But seriously. I saw a 10" blonde naso in a 120 short (48x24x24) and it was certainly the saddest looking naso I've ever seen.