I bought a cheap one at Walmart (Intex brand?) in the twin size a few months ago and used it for the 2nd or 3rd time last night.
I don't remember the exact dimensions, but its approx 74" x 39". Width is not an issue; you'll have several inches to spare. Length is tight. I air it up from the back hatch with the front of the air mattress resting on top of the folded back seat. With the back hatch closed, I open the back door and jam the mattress down so that it lies flat. If you're about 5'9" or shorter, it'll be perfect.

I'm a hair under 6' and I can't stretch out fully in the 4runner unless I lay diagonally which doesn't work with the air mattress. I'm going to try one of the more compact backpacking mattresses and see if it'll fit diagonally.