The hotels don't usually publish their total square footage, just the square footage of the casinos and the numbers of rooms. The MGM Grand when it opened in 1993 was considered the biggest hotel in the world with 5044 rooms. They have subsequently closed the theme park and built several more towers . Currently it is:
-The main hotel building, with 5,044 rooms
(4,293 rooms and 751 suites)
-The three The Signature at MGM Grand towers each with 576 suites
-SKYLOFTS at MGM Grand with 51 lofts
-The Mansion at MGM Grand with 29 villas
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The 30-floor main building is 293 feet high and features five outdoor pools, rivers, and waterfalls that cover 6.6 acres, a 380,000 square foot convention center, the MGM Grand Garden Arena, CBS Television City, and the Grand Spa. It also houses numerous shops and night clubs, 16 restaurants, two food courts, and the largest casino in Clark County, which occupies 171,500 square feet .
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The Sands corporation which built the Venetian hotel in 1999. Combined with the adjacent Sands Expo Convention Center and The Palazzo Hotel and Casino Resort, The Venetian is a part of the largest hotel and resort complex in the world --- featuring 7,128 hotel rooms and suites.
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May 3, 1999 = Venetian opens with 3047 suites & rooms
June 27, 2003 = The Venezia at the Venetian opened as an additional tower, adding 1,013 suites and a new wedding chapel.
January 17, 2008 = The Palazzo with 3,068 rooms opened.
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Because the Venetian complex opened in stages over 9 years and has two seperate checkin counters and marketing campaigns, some people call it a hotel complex rather than a single hotel. Many people still defer to the MGM Grand as the largest single hotel.
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Keep in mind that the MGM-Grand is owned by the MGM-Mirage corporation which already owns ten hotels on the strip with 38,000 rooms (about half of the rooms on the strip). They are planning to open a complex next year with another 6350 rooms spread among 4 hotel or condo complexes.