The contenders are:
Queen Mary 2
Carnival Corp. GRT 148528 LOA: 1132ft (345m) Passenger capacity: 2592 double occupancy.
Interestingly enough our biggest box boat in service is 1305 ft (398m) long. Allure is probably one of the biggest ships out there, although she is to be outstaged by an even bigger ship one of these days.