Barges are huge. Passenger ships can be giant too. Look at the Queen Mary! It's over 1,000 feet long! Barges can be over 1,000 feet long as well.. There are others, but those are the main two.

US Navy Battleships and aircraft carriers are both over 1000 feet long, but I believe the largest by displacement (weight of the ship measured by how much water it displaces) has to go to petroleum supertankers. The winner, from "Ask Yahoo": "the oil tanker Jahre Viking, which logs in at 1,503 feet long and 226 feet wide. It boasts a gross tonnage of 565,000 tons! "

ships can be measured in many different ways. 1) the amount of cargo they can carry (the largest tanker can carry about 585,000 metric tons.). this lifting capacity is called deadweight cargo carrying capaicty (dwcc) 2) by the weight of the actual ship. (net weight or net registered tonnage). passenger ships usually weigh more than tankers or dry cargo ships as they have more 'stuff' on them. 3) displacement (how much water they displace when they are put in the water).. this is hardly ever used. 4) gross registered tonnage. this is a formula of the space inside the ship, in cubic feet, divided by 100. the largest ship in cubic feet are woodchip carriers. woodchips are 'light' in cuft to weight ratio so they need a lots of cubic feet to move each ton (about 100 cubic feet per metric ton.. versus say.. iron ore which needs about 6 cubic feet per metric ton!) the largest ship in dwcc is the m/t 'Seawise Giant'.. the 585,000 dwcc tanker mentioned above. the largest ship in passengers is the Staten Island Ferry (no joke) the largest car carrier lifts about 7,500 mid-sized cars. the largest dry bulk carrier is about 380,000 metric tons dwcc the largest container ship is being built now.. they can carry about 9,000 containers (but it depends how much each container weighs). the largest ship to pass through the panama canal is, i believe a usa air-craft carrier. don't know which one.