With a gross tonnage of 226,963 GT, she is the third largest passenger ship in the world, larger than her older sisters Oasis of the Seas and Allure of the Seas, but surpassed by her newer sisters Symphony of the Seas and Wonder of the Seas.

As of 2026, Harmony of the Seas remains one of the largest cruise ships ever built, though it has been slightly surpassed by its newer sister ships in the Oasis and Icon classes. When it launched, it held the title of the largest passenger ship in the world. It boasts a staggering gross tonnage of approximately 226,963 tons, measures 1,188 feet in length, and can accommodate over 6,600 guests at total capacity, supported by a crew of 2,300. To put its size into perspective, it is nearly 100 feet longer than the Eiffel Tower is tall. The ship is divided into seven distinct "neighborhoods," including the iconic Central Park with real trees and the Boardwalk. It features 18 decks, 23 swimming pools, and the "Ultimate Abyss," a 10-story slide that is one of the tallest at sea. While the newer Icon of the Seas is now the largest, Harmony is still a "city at sea" that offers a level of scale and variety that few other vessels can match.