Originally, Walt Disney started with 160 acres in Anaheim. Through a series a acquisitions, the site was expanded, especially in recent years. I don't know how much land Disney owns in Anaheim, but I wouldn't be surprised if it's still only around 400 or 500 acres.
Walt Disney secretly acquired around 27,400 acres in Florida in the 1960s. Central Florida land was very cheap back then, and Walt had ambitious plans for a city called EPCOT. Today, WDW it's somewhere around 30,000 acres. The town of Celebration will eventually use around 5,000 of the acres.
Disneyland Paris sits on a site that's somewhere around 4,800 acres (although I've seen numbers ranging from 1,500 acres to 5,000 acres). Euro Disney SCA, the company that owns Disneyland Paris, is actively developing the land.
In Virginia, The Walt Disney Company planned to build Disney's America on a 3,000 acre site.
The theme parks themselves vary in size. DCA is only 55 acres, but that's because Disney has so little land in Anaheim. But figure 100 to 150 acres for a theme park, not including the parking lot.
So "3,500 acres in Eolia, Missouri" is in the right ballpark for a theme park resort development.
But I don't think it will happen.