There are different tanker sizes used for the international transportation of oil, from a modest coastal tanker to a VLCC or ULCC Supertanker. The common rule is that the volume that can be carried in a tanker increases as a function of the cube of its length. For instance, a ULCC is about twice the length of a coastal tanker (415 meters versus 205 meters), but can carry about 8 times the volume (400,000 deadweight tons versus 50,000 dwt).
The largest ship ever built was an oil tanker; the Knock Nevis was built in 1979 (launched as the Seawise Giant) with 565,000 dwt, a length of 458 meters, a beam of 69 meters, and a draught of 25 meters. The ship was so large that no port could accommodate it fully laden, so it was loaded and unloaded while anchored offshore. In 2004, the Knock Nevis was converted to a floating storage and mooring unit off the coast of Qatar until 2009, when it was scrapped. As of 2010, only 12 tankers above 320,000 dwt remained. Of this, only two “true” ULCC of around 430,000 dwt are left in operation, the TI Europe and the TI Oceana, which were part of a group of four ships constructed between 2002 and 2003. The other two ships, TI Africa and TI Asia, were converted into floating storage and mooring units in 2010. Oil tankers have a commercial life expectancy of about 30 years.
Because of their mass, tankers have large inertia, making them very difficult to steer. A loaded supertanker could take as much as 4 to 8 kilometers and 15 minutes to come to a full stop and has a turning diameter of about 2 kilometers. Among the main tanker classes are:
| Class | Length | Beam | Draft | Overview |
| Coastal Tanker | 205 m | 29 m | 16 m | Less than 50,000 dwt, mainly used for transportation of refined products (gasoline, gasoil). |
| Aframax | 245 m | 34 m | 20 m | Approximately 80,000 dwt, which is the AFRA standard (Average Freight Rate Assessment). This standard was established to standardize contract terms with well-defined ship capacity. |
| Suezmax | 285 m | 45 m | 23 m | Between 125,000 and 180,000 dwt, originally the maximum capacity of the Suez Canal. |
| VLCC | 330 m | 55 m | 28 m | Very Large Crude Carrier. Up to around 320,000 dwt. Some can be accommodated by the expanded dimensions of the Suez Canal. The most common length is in the range of 300 to 330 meters. |
| ULCC | 415 m | 63 m | 35 m | Ultra Large Crude Carrier. Capacity exceeding 320,000 dwt. The largest tankers ever built have a deadweight of over 550,000 dwt. |