In general when people talk about the size of their TT they mean the size of the box length rounded up to the nearest foot.
Things like the curvature or slant of the front or back, a V pointed end and such do not matter. The most common use for this information is to tell the campground what size TT you are bringing in.
Knowing the exact volume of your TT is kind of like asking "How many angels can dance on the head of a pin?" an interesting mind puzzle but not really all that important in day to day activities.
If of course you are custom fitting a cover, or measuring for a garage, or aligning a number of TTs for storage and such, then the exact bumper to ball receiver tip and fender skirt to fender skirt bulge can be important.
Most often saying I have a 25 or 28 footer etc. is all the information the other person needs.