Before it opened, Fallsview's developers said that it would be 90,000 square feet. As it was being built, its designers added a mezzanine and outdoor pool which brought the total up to 125,000 square feet. However, it doesn't have the number or variety of water attractions, nor the marquee slides and rides that indoor water parks of similar size offer.
The Kalahari Ohio's indoor waterpark is 173,000 square feet. How, then, can the Fallsview people say that their waterpark-hotel complex is North America's largest? Because, they claim the Fallsview complex includes 1200 hotel rooms--the most hotel rooms of any indoor water park resort. Whew! That's a stretch. So, the park is not the largest in square footage; it's the largest in affiliated hotel rooms.
Regardless, both the Fallsview and the Kalahari may have to bow to the new king of North American indoor water park resorts; when, and if, Las Vegas Wet opens, it would feature 350,000 square feet of water park fun under one roof--and 1400 hotel rooms.
The key word in all of the claims above is "resort." There is another indoor water park now open in North America that's larger than the Fallsview or the Kalahari properties. But it's a standalone water park that is not part of a resort.