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Perhaps the best thing to do would be to say that one ship is the biggest in terms of length; another in terms of gross tonnage, and so forth.

You're absolutely right with regard to Queen Mary 2's length; as per Wikipedia:

'At the time of her construction in 2003, the QM2 was the longest, widest and tallest passenger ship ever built, and at 151,400 gross tons, was also the largest. She lost that last distinction to Royal Caribbean International's 158,000 gross ton Freedom of the Seas in April 2006, but QM2 remains the largest ocean liner (as opposed to cruise ship) ever built, and her height, length, and waterline breadth are unsurpassed by any other passenger ship.'

The full article is found here: 'http://en.wikipedia.org/wiki/RMS_Queen_Mary_2' I won't quote any more due to copyright restrictions.

Best wishes,

Mark.