Semi-trailer lengths are limited by law. Generally, California limits are smaller than Nevada, so we concentrate on California. Generally, semi-trailers are limited to 53 feet, with an exception for longer if the tractor is so short that the overall length is no more than 65.HOWEVER: Flatbeds are rarely more than 48 feet
, and older ones can be 45 or even just 40. So you will emphasize you need a 48-footer. There are some provisions for freight extending past the rear of the trailer, but this is so complicated it will need to be researched after we know the length of the tractor. Off the top of my head, there is an absolute maximum overall length of 75 feet, if it is the freight that makes it that long.
It is possible to get a permit for oversize freight, but only if the freight cannot be divided into smaller pieces.
Generally, shipper and consignee are responsible for taking the freight on and off the trailer, while the driver is responsible for tying the freight down. In some cases, the shipper ties the freight down, to be sure to comply with the requirements of the freight.
You might want to be aware that flatbed trailers are often arched a bit -- that is, the deck is curved, highest in the middle. This is so the trailer will not sag below flat if maximum weight must be loaded in the middle of the trailer. So you may need to provide lumber to shim the freight level, depending on what you are shipping.
Weight may also be a concern. Very roughly, an 18-wheeler can carry up to something like 48.000 pounds, but that must be evenly distributed. The maximum gross weight is 80.000 pounds. "Spread axle" trailers can carry a slightly larger percentage on the tail, but the gross is still 80.000.
Hope that helps.