I went to Lowes last week to pick up a piece of countertop for a project and snapped this photo of how it ended up loaded in my car. The countertop is 6 feet long and 24″ wide, but I was pretty sure it would fit in my Jetta. Since it was raining when I left, one of the clerks volunteered to help me load the top, if I “pulled my vehicle [truck] up front”.
He looked a bit dubious when he saw the vehicle I intended to load it in, but I was pretty sure it would fit. The bottom line is that it fit, with the trunk shut, though I wouldn’t want to ride in the front passenger seat. With that seat slid all the way forward and completely vertical, the distance between the trunk lid and the back of the seat is exactly the required 6 feet. There is an additional 3+ feet to the windshield, if you had something that could reach over/around the seat, like 8′ 2x4s (Which also fit fine, up to about 10 of them).
I think my next car (someday) will probably be a wagon of some sort, to better accommodate foolishness like this. Notably, though, a few days after this picture was taken, I hauled a number of pre-built cabinets in my car without issue. Hint: With the rear seat in place, height through the rear door is about 29 inches. If you flip the seat-pans forward (like you might before laying down the back seat), it gives an extra 6″ of height.