The author means ships so large they don't fit any locks - to traverse oceans they must go around the Cape of Good Hope (South Africa), or Cape Horn (South America). Most of these are freighters or oil tankers, though some new cruise ships are very large too. Here's the Wikipedia photo of the largest cruise ship, the Oasis of the Seas.
Ships can be described by the size of the locks or canals that they can fit through, or the ports they fit. For example, HandyMax means the biggest cargo ship that fits almost any port.
Besides being the right length, width, draft (amount below water) and air draft (height above water), ships must be trimmed so they sit completely level in the water. Their captains must adjust for the fact that most locks are filled with some fresh water which is less buoyant than salt water. And if they are going under a bridge, steaming full-speed-ahead actually lowers the ship by a few percent!
|Length||1187 ft (360m)||965 ft (294m)||1,200 ft (366m)||1,400 ft (427m)||1400+ ft|
|Width||208 ft (64m)||107 ft (32.6m)||160 ft (49m)||164 ft (55m)||200+ ft|
|Draft||30 ft (9m)||39.5 ft (12m)||50 ft (15m)||62 ft (18.5m)||50+ ft|
|Air Draft||213 ft (65m)||190 ft (58m)||201 ft (61.3m)||223 ft (68m)||200+ ft|
|Displacement||100,000 tons||80,000 tons||120,000 tons?||150,000 tons||≥200,000 tons|
The largest (longest) ship ever built was known as Knock Nevis. She was 1504 ft (485m) long, 226 ft (69m) wide, and had a draft of 81 ft (25m). That made her too big even to go through the English Channel! Knock Nevis was built in 1979 and after living a short, rough life, was beached last month. She's being dismantled and scrapped right now.