To some extent, it depends on why you're asking.
If it's to fit into a camp site or your driveway, the length of the entire trailer rather needs to be no more than the space available.
If it's for DOT registration or taxation purposes, go by whatever measurement they use, no matter how convoluted or nonsensical.
If you're selling, go with the longest measurement you can find (for a largish trailer) or the smallest one (for an especially compact trailer), whichever would attract the most money.
If you're buying, go with the opposite of the measurement if you're selling.
If you're a manufacturer and coming up with a model designation, pick any old number out of a hat.