I had two carriers in mind when I started looking for information on exact square footage. Since I was not able to find any square footage listings I am going with my first instinct and say it was the Lexington class ships USS Lexington CV-2 and USS Saratoga CV-3.
As these hulls were converted from battle cruisers I believe they were the largest until the Midway class carriers post war.

__________________
Bill Murray
Why do we press harder on the remote control when we know the batteries are getting weak?