It depends on who's doing your PR
. Foxwoods Resort Casino
in Connecticut claims to be it. They have 340,000 sq ft. of gaming space, 1,416 rooms and suites, and the total size of the Foxwoods complex is 4.7 million square feet. And are planning to expand by 2 million square feet which will include an additional hotel tower, a 4 story parking garage additional gaming space, resturantants, and entertainment options.
in Las Vegas also lays claim to the title as well, but in reality only if you throw in the Sands Convention Center, which was the only thing not imploded when the Sands was imploded to make way for the Venetian. They can lay claim to being the world's largest hotel/convention center, but their casino, at 110,000 square feet is smaller than the casino space @ MGM Grand (171,000 square feet).