I can only speak for the laws in Calif. for the 53' trailers, they measure from the kingpin to the center of the front axle on the trailer, not the rear of the trailer. But that is only for hauling one trailer. For doubles, you can go to '75 feet overall length, but the trailers can be no longer that 28.5' and you have to use a tractor to haul them ( mdt or hdt). For doubles, you have to have a commercial license. If your FW is over 10k, but under 15k, you need an endorsement on your regular DL. If it is over 15k, and not hauling livestock, you have to have a non-commercial class A. If you have a horse trailer with living quarters that is over 15k, you have to have a commercial DL. Calif wil accept your home state DL if you combination of vehicles is valid in your home state with the class of DL you posses is good for that combo in your home state.
However, they will not accept the length and/or weight that is valid in other states if they are not permitted here. For example, both Nevada and Oregon allow two 53' foot trailers and Oregon allows triples. However, neither are legal in Calif regardless of what your Nevada or Oregon commercial DL allows you to operate in those states.
Clear as mud?