The length is the actual length of the trailer, bumper to hitch. Model numbers do NOT represent box size. In fact hardly any maker post measurements of the box because it doesn't matter. Campsite length is the biggest factor in making reservations. You want to make sure your camper will fit the entire camper, as the box can't be removed off of the frame. :)
Tongue lengths are usually 3.5' long so you can get a rough guestimate of the box size that way but measuring yourself is the best if you NEED to know the box size.
When people refer to camper size, it's the full bumper to hitch length. That's the only measurement that matters for fitting your RV in storage, your driveway or the campsite.