At a time when store closures are accelerating and struggling malls pockmark the country, county commissioners in Florida have approved a plan to build what would be the largest mall in the U.S.

American Dream Miami would also be the most expensive mall ever built, according to Canadian developer Triple Five Worldwide Group of Cos. The 6.2-million-square-foot retail and entertainment complex would cost an estimated $4 billion, Triple Five says.

The...