Walt Disney World is approximately 43 sq miles. It takes up 27,000 acres of land. "It has been described... as a land area approximately the size of the city of San Francisco, or twice the size of the island of Manhattan" Since The World Began
It consists of 4 major parks, Magic Kingdom, home of Cinderella's Castle, Epcot, Disney/MGM Studios and Animal Kingdom. It also is home to Blizzard Beach and Typhoon Lagoon Water Parks, Disney Village Marketplace, Pleasure Island, Disney Westside, Fantasia Gardens and WinterSummerland Miniature Golf, Disney's Wide World of Sports Complex, the Walt Disney World Speedway, 99 holes of golf on 5 championship courses, 19 Resort Hotels and Vacation Club Villas, and campground, and has its own utility plants, waste water treatment plants, tree farms, telephone system, and roadway systems.
Basically, when you enter WDW, you are entering a well run city.