The size of a D Anchor Shackle solely depends on the diameter of the ship chain that it is attached to. The smallest Anchor D Shackles that are used in the ship industry commonly have a diameter between 12.5 and 50 mm. These are used by fishing boats usually. The medium sized ones start at 52 and end at 100 mm of diameter, and are used in ferries and cruises. The largest ones, the big sizes, start at 102 mm and can be up to 175 mm diameter, and are used by the largest ships in the World, such as oil tankers, aircraft carriers and cargo ships.