They're slow growing and don't get very big. You're unlikely to see one bigger than about a foot in an aquarium. As such, a 6' by 1.5' footprint would work, but a 6' by 2' would be better as I assume that it probably won't be solo.
I just picked up this article (i wanted the picture and the article was a freebie i did not know about! ) figured it might be of interest, the picture is from 1841, so i guess the article is aswell