How Consumer Reports tests cars:

Trunk and cargo space

For cars with an enclosed trunk, we measure its usable volume with a set of typical-sized suitcases and duffle bags. For cargo-oriented vehicles such as hatchbacks, station wagons, and SUVs, we use an expandable rectangular pipe-frame "box." We enlarge it enough to just fit through the rear opening and extend into the cargo bay as far as possible without preventing the hatch from closing. Cargo capacity is the volume enclosed by that box. For pickup trucks, we measure the volume of the load bed up to the top of the side rails.