At dock in its vast custom-made port in Fort Lauderdale, Florida, the Oasis of the Seas gives you little clue that you are looking at an ocean-going vessel. The dozen above-deck storeys of the world's largest cruise liner, with their identical rows of curved glass balconies, make it all but indistinguishable from the high-rise condos and office blocks that line the coast road south from Miami.

On board, the illusion is heightened. This is a ship designed to appear anything but. It has at its heart a central park bigger than a football pitch and recently planted with 12,000 trees and shrubs. Contained on all sides by the steep stacks of cabins, the park is crossed by stone-paved paths and walkways leading to cafes and bars. Sitting outside any of them, it is hard to feel anything other than landlocked.

The ship is sold as an urban experience, a city on the ocean, and on a preview voyage out towards the Caribbean that is what most people seemed to enjoy: getting a cappuccino in one of the ship's seven distinct "neighbourhoods" and indulging in some serious consumption.

How big is big? Five times the tonnage of the Titanic, the Oasis is longer than any aircraft carrier in the US fleet. It is half as big again as the O2 centre. Stand it on its end and it would look down on Canary Wharf's towers.

There is no accessible place on the ship from which you can view both its bow and stern, so once you are aboard you have to find your own ways to understand its scale. You can jog its perimeter, each lap almost half a mile. You can harness yourself into a zipwire and fly nine storeys above the main deck. Or you can attempt the world's most meandering offshore pub crawl: 37 bars and restaurants spread over 16 levels, including an English theme pub, in which smoking is allowed.

The Oasis, which is owned by Royal Caribbean International, makes its maiden public voyage next week. It will typically carry 5,400 passengers (though the capacity is upwards of 6,000), served by 2,291 staff: 40% bigger than any other "megaliner" in the expanding cruise market. Because of its size, there are very few ports at which it can dock; the shipping line has confronted this difficulty by creating its own "resort destination" in Haiti, a purpose-built docking station called Labadee; and the Jamaican government has invested heavily in upgrading the harbour at Falmouth, in Jamiaca, to allow it to discharge passengers there.

Excluding these efforts, at a cost of £800m the Oasis is the most expensive commercial vessel ever constructed. It was built in Finland, and sailed into Florida last month. In order to leave the Baltic, its stumpy funnel had to be retracted and the ship was required to travel extremely slowly, so that it sat low in the water. Even so, it only just scraped underneath the Great Belt Fixed Link bridge, 65 metres (213ft) over the water, by about 50cm.

Crossing the Atlantic, the ship's stability was tested in a near hurricane-force storm. Given its towering superstructure, the Oasis had to be of an unprecedented width to prevent it being top heavy – the park at its centre is there to help with balance, as much as anything. It is by no means a sleek craft.

It is also sailing very much against the economic current. Royal Caribbean has been the prime mover in the ongoing arms race of liner size, mostly played out in the docks of Miami, where the US cruise industry is based. This decade, the company has three times created the world's largest passenger ship. Up to now, this biggest-is-best philosophy has always worked.

The Oasis was planned well before the global economic downturn, however. And that downturn has hit Florida particularly hard. The ship is not yet fully booked for next year's voyages, but Royal Caribbean expects it to carry 40% of the line's British passengers.

After the Titanic, no one is going to claim that the biggest liner in the world is unsinkable. But its owners are no doubt trusting that the Oasis of the Seas is too big to fail.

Ship stats

The Oasis of the Seas can carry a maximum of 6,292 passengers plus crew of up to 2,291.

Its 16 passenger decks contain four pools and 10 whirlpools, a rock-climbing wall, a basketball court, a miniature golf course, an ice skating rink, a shopping promenade, a fairground carousel, a central park with 12,000 live plants and trees, and a 1,380-seat theatre.

It has 37 bars and restaurants, 4,100 toilets, 42 lifts and 4,500 air conditioning units.

Seven-night cruises start at $1,049 (£628) per person for an inside cabin and go to $16,659 per person for the two-storey Royal Loft suite, which includes a baby grand piano and private 843 sq ft balcony.

• In the Observer this Sunday: Tim Adams's verdict on the world's biggest passenger ship

• This article was amended on 26 November 2009. The original said that the liner was expected to carry 40% of Britain's 1 million-plus cruise passengers. This has been corrected. The difference between two passenger-capacity figures has also been clarified.