So this is in fact the biggest ship that has even been built,,,,

Pioneering Spirit is the world’s largest platform installation/decommissioning and pipelay vessel built for the Allseas company.[1] The ship is also by far the world’s largest vessel overall ever constructed in terms of its gross tonnage of 403,342 gt, as well as its breadth (123.75 m/406 ft) and displacement (900,000 metric tons).[2] The main construction was done at Daewoo shipyard, Korea and final completion will be in the Netherlands. Main diesel power comprises eight MAN 11.5 MW engines and one 4 MW harbour engine with two engines each in four separate engine rooms[3] with 12 Rolls Royce 5.5 MW thrusters.[4] The vessel was designed by a Finnish engineering company Deltamarin. Allseas¹ has committed to build Pioneering Spirit at a cost of US$1.7 billion. In an interview with De Telegraaf”[5] chairman Heerema named a price of €2.4 billion for the 382 metres (1,253 ft) long vessel. AllSeas has committed to building an even larger version of the same design, which will be operational in 2020.[6]