What is the longest eighteen wheel trailer length?

Arizona, Kansas, Louisiana, New Mexico, Texas, and Wyoming will allow you to pull 57 foot trailers. Texas and Louisiana will allow you to pull 59-foot trailers that were licensed prior to December 1, 1982. All states will allow you to pull 53-foot trailers. Although the technically correct answer is 59 feet, the fact that the only run you can actually make in a 59 is from Texas to Louisiana and the very limited range of states a 57 can be used in means the longest practical trailer is 53 feet long.