http://en.wikipedia.org/wiki/Containerization

There are five common standard lengths, 20-ft (6.1 m), 40-ft (12.2 m), 45-ft (13.7 m), 48-ft (14.6 m), and 53-ft (16.2 m). United States domestic standard containers are generally 48 ft (15 m) and 53-ft (rail and truck). Container capacity is often expressed in twenty-foot equivalent units (TEU, or sometimes teu). An equivalent unit is a measure of containerized cargo capacity equal to one standard 20 ft (length) × 8 ft (width) container. As this is an approximate measure, the height of the box is not considered, for instance the 9 ft 6 in (2.9 m) High cube and the 4-ft 3-in (1.3 m) half height 20 ft (6.1 m) containers are also called one TEU.

The maximum gross mass for a 20 ft (6.1 m) dry cargo container is 24,000 kg, and for a 40-ft (including the 2.87 m (9 ft 6 in) high cube container), it is 30,480 kg. Allowing for the tare mass of the container, the maximum payload mass is therefore reduced to approximately 22,000 kg for 20 ft (6.1 m), and 27,000 kg for 40 ft (12 m) containers.

The original choice of 8 foot height for ISO containers was made in part to suit a large proportion of railway tunnels, though some had to be deepened. With the arrival of even taller containers, further enlargement is proving necessary.