If all you remember is the opening line identifying the book is going to be fairly difficult. With that being said The Greatship by Robert Reed is about a very large ship and I vaguely recall the beginning of one of them describing the ship in a somewhat similar way to what you remember.
Since the beginning of the universe, the giant starship wandered the emptiest reaches of space, without crew or course, much less any clear purpose. But humans found the relic outside the Milky Way, and after taking possession, they named their prize the Great Ship and embarked on a bold voyage through the galaxy’s civilized hearts.
Larger than worlds, the Great Ship is laced with caverns and oceans, scenes of exalted beauty and corners where no creature has ever stood. Habitats can be created for every intelligent species, provided that the passengers can pay for the honor of a berth, and the human captains make the rules and dispense the justice in what soon becomes thousands of alien species joined a wild, unpredictable journey.