From the article:

Furthermore, some of the world's main waterways such as the Suez Canal and Singapore Strait also restrict the maximum dimensions of a ship that can pass through them

There are limits to how big a ship can be and still transit a specific route, usually defined by the radius of curves in a canal or the length of a lock.

In this case, 400m is the max length required for the Suez (https://en.wikipedia.org/wiki/Suezmax)