Long before the hoteliers in Wisconsin Dells kicked off the indoor water park resort phenomenon and started battling it our for bragging rights, the West Edmonton Mall in Alberta Canada, a humongous ode to consumerism, opened its humongous World Waterpark. At 5 acres, or roughly 225,000 square feet, the mall's water park is larger than any other indoor water park in North America -- with the prickly exception of the Wilderness in Wisconsin Dells.
If you combine the three separate indoor water parks at the Wilderness, they total 250,000 square feet. The World Waterpark is the largest indoor water park, by far, under one roof. It is not, however, a water park resort. A standalone water park, it is one of the attractions at the West Edmonton Mall (which also includes an indoor theme park). The mall, however, does offer some onsite hotels, so it could be argued that the mall itself is a resort.
A mega indoor water park resort is under development in Las Vegas that, when and if it opens, would be larger than the World Waterpark.