USS America

It likely was the largest ever since the largest warship sank in combat.
It was the Japanese battleship Yamato which was 69,990 tons when fully loaded.

http://www.cdnn.info/news/industry/i050522a.html
USS America 1048-ft aircraft carrier sunk off North Carolina
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NORTH CAROLINA (22 May 2005) -- The retired aircraft carrier USS America is on the bottom of the Atlantic Ocean, sunk by the Navy in a series of explosive tests that upset some veterans.
The 84,000-ton, 1,048-foot warship that served the Navy for 32 years rests about 60 miles off the coast and more than 6,000 feet down, according to Pat Dolan, a spokeswoman for Naval Sea Systems Command.
She did not give a location, but the Navy previously said the explosions would take place off North Carolina.
Dolan said the America went down May 14, finally flooded after the series of explosions over 25 days. No announcement was made at the time.
No warship this size or larger had ever been sunk, and plans to sink the America caused controversy.
"Not a day goes by that I don't think about it," said Lee McNulty, president of the USS America Foundation, which wanted to turn the ship into a museum. "Of all the carriers, that one should have been saved, just for the name America."
The America launched warplanes during the Vietnam War, the 1986 conflict with Libya, the first Gulf War, and over Bosnia-Herzegovina in the mid-1990s.

The Navy said in March that the explosive tests would provide valuable data on survivability for the next generation of aircraft carriers, which are now in development.
Since its decommissioning in 1996, the America had been moored with dozens of other inactive warships at a Navy yard in Philadelphia.