The largest cruise ship as of 2021 - the Symphony of the Seas is an Oasis-class cruise ship built by Chantiers de l’Atlantique for Royal Caribbean International.

The Symphony of the Seas is 362.12m long, 72.5m tall (above waterline) and 65.m wide, and features 18 decks and 2,759 rooms. It can accommodate up to 6,600 passengers, with 2,200 operating crew

Here's an interesting comparison using the data comparing tool - Symphony of the Seas as the largest cruise ship in 2021 versus Titanic the largest cruise ship in 1912.