I am fairly sure that the Coral Reef lagoon (no side viewing) is that big. They used to have it (or something similar) set up at the SA hatchery with some sharks and a few fish in it a year or three back. I am not sure if it is up and running yet, but somebody was working on it a week or two ago.
Aquatic Marine's big tank is only 1200G, but it is a big acryllic tank and not a lagoon.