As measured by square feet of gambling space, here they are. This comes as a surprise to me, as I've barely heard of the two Oklahoma casinos in the top five.
Top Five U.S. Casinos
CasinoLocationSquare Feet Winstar Thackerville, OK 519,000 Mohegan Sun Uncasville, Connecticut 364,000 Foxwoods Mashantucket, CT 344,000 San Manuel Highland CA 220,000 Riverwind Norman OK 216,000