You were suplied with incorrect information. In fact I'm being a easy on the tank size. See this (offsite) -
http://www.liveaquaria.com/product/prod_display.cfm?c=15+2971+330&pcatid=330 Note that they recommend 180 gal.
Size of the fish has little to do with it, since the fish needs the "running room". I also feel that you are better off with a longer tank, a 6 ft long 125 gal tank would be better than a 4 ft long tank of the same volume.
Now people have gotten away with using smaller than recommended tanks, but it's risky. I'd say consider something like a 125 or 150. It will be longer than a 90, and cost somewhat more to set up, but you'll be doing right by the fish.