I need to transport some wood and it's nice to know the dimensions I can carry in the Model S. I have some large projects coming up around the home including kitchen cabinets, a bench seat, and a storage bed. I took rough measurements and figured I could carry 6'x3' (sorry metric folk) sheets. So I verified it by getting a 1/4" 4x8 plywood I needed for a repair project and had the store cut it down to size with two cuts. Ripping 1' off the length and 2' off the width to make 4 pieces with the two free cuts the store offers.
Here are the pictures. The large 6'x3' sheet fit with ease. I originally thought the center console would get in the way which is why I figured 6' max but I could have had another 6" to the length because the rear seats don't fold perfectly flat and make the sheet incline over the center console by a couple of inches. There was also clearance for another 1" to the width between the tire wells. I'm 6'1", all leg and I didn't need to move the seat up any. The last picture from the side I butted the sheet up to the rear seat where I normally drive and you can see the clearance over the center console.
I'll probably make a ramp out of the leftover sections to make loading the heavier 3/4" sheets easier.