Trailers are rated by two criteria, length and width.
In general trailers of all types range from 20 feet in length to as long as 60 feet. There are also tandem trailers, that will combine two, and sometimes three, shorter trailers.
102-inch wide dry-vans are most used with extra large or extra wide loads. A standard trailer is only slightly wider than the truck that pulls it, but 102 wide vans are wider and change the dynamics of driving the vehicle, especially during turns.
In the United States drivers are required to possess a Commercial Drivers License (CDL) to drive a dry van with more than 2 axles. Other types of vehicles, such as tankers, 102 wide vans, Hazmat, or tandems require additional testing, both driving and written, to be endorsed to operate these vehicles.