Is the louvre an art museum?

In terms of floor space (square ft/m), no.

That said, it is the most visited annually (some 8,880,000 people).

The largest art museum in the world is the Metropolitan Museum of Art in NYC (aka the Met). The single building is *over* 2,000,000 million square feet (more than 185,806.08 square meters). It is also the 2nd most visited annually with 6,004,254 visitors.