Welcome to the forum, JuliusCat. Just went out to garage to get some dimensions for you. With the seats folded and the cargo cover removed here's what I got.
Width- 40" (between wheel wells)- 44" (max.)
Height- 28.5" (max.)
If you were to put big boxes back there the max. sizes would be about 43"x40"x28.5" or 50"x40"x22".
With the seats up and cargo cover in place you get about 40"(width)x17"(height)x28"(depth).
Of course the width between the rear doors is a bit more and there are a couple nooks in the back. These measurements are with the underfloor storage option, so you'd get a bit more height in the back without it. Hope this helps your decision.