It depends on the person. Use the model number, the model number is the living length, the overall length will be 3-4'
longer.
My 26BHS is a 26'-class trailer, but tip-of tongue to end of spare tire is just over 30'.
On used listings, it's really up to the person's preference how they list the length. If you get the model number, it's really easy to look it up online and find out exactly what the manufacturer says it is, but simply adding 4' will get you in the ball park too.