If you mean the volume rather than area you need to fill the space with something that will not damage the interior of the space and will not escape, so water is out of the question for both reasons. You could use shredded paper, rags, newspaper, etc., which would be easy to remove afterwards. Pack as much space as possible. Then empty the filling material into a box of regular dimensions that you can measure. If there's too much material, fill the box to capacity and pack to the same density as you used to pack the cargo space. The volume of the box (length times width times height) gives you the volume of material removed. Empty the box, and then transfer as much of the filling as you can into the box from the cargo space until the box is full again or you've removed all the filling. If the box isn't full you can measure how far the filling rises on the box and so determine the volume. Best to measure the distance from the top layer of material to the top of the box, and check that this distance is the same all round, then subtract this measure from the height of the box.

Keep a note of how many times you've filled the box and what the volume of the last quantity of filling was. Multiply the volume of the box by the number of times you filled it and add the final volume of the partially filled box.

This should give you an idea of the capacity as long as you packed the box to about the same density as you packed the cargo space. In practice of course, you could never fill the entire cargo space with stuff and a few additional measurements such as maximum height, width and depth may be necessary for ensuring goods will fit into the cargo space.