How long can a trailer be?

East of the Mississippi, trailers come in 4 standard sizes.
28 foot pups, used in double and triple configuations.
45, 48, and 53 foot dry vans. Most states do not permit these trailers to be pulled in tandem.

Anything longer than a 53 foot trailer requires special permits as well as flag cars to lead and follow the truck down designated routes.

There are 13 states which permit two 53 foot long trailers to be pulled in a double configuration. These combinations are called "Turnpike Doubles" and cannot leave highways/interstates. There are often drop yards on the sides of the highways in permitting states to break these sets apart.

The 53' limit is due to traffic congestion and the general lack of room needed to make turns wide enough for these trailers to not run over curbs.

West of the Mississippi, limits vary by state. Some states allow trailers upto and over 70 feet in length. It is the trucker's duty to know the regulations for each state that he drives through. His atlas generally has a breakdown of the laws for a quick look.

The 53 ft (16.2 m) limit on single trailers applies in Canada, although each province has its own regulations. Multiple trailer configurations are also permitted, with the maximum combined length of the trailers ranging from 20 to 25 m (65.6 to 82 ft), depending on the province.

You will find that intermodal containers are produced to fit common trailer lengths in various countries. Transport trucks may carry several containers at once, depending on their size. Container lengths include: 10, 20, 24, 28, 40, 44, 45, 46, 48, 53, and 56 feet.

There are no actual limits imposed by any State other than a maximum limit without a special permit. States realize that in special circumstances, something may need to be moved, which requires a special vehicle/trailer combination that exceeds the size limitations of what is normal. States are more interested in the GVW or gross weight of a vehicle than size limitations, which are few and far between. Most states are mandated that anything hauled by a semi/trailer cannot impose any limitations of anything under 48' or 28' for each of a double or triple combo. This does not necessarily mean that they exclude these, just that they may require a special permit to operate them, including fees for things like a highway patrol escort. Many commercial carriers haul mobile homes which exceed the 48' limit all the time, it is just they have to file and pay for a special permit with or without state escorts and other rules. You would have to contact your states local BMV to find out a state specific answer to your question. For information on what the Federal government says on this, go here:

http://www.fmcsa.dot.gov/rulesregs/fmcsr/regs/658.13.htm