You know, there's another thing you should know about the Prius and the Prime - the front seats can fold down in front of the back seats. You have to take the head rests off and it is a little bit of a wrestling match, but it fits. That makes the interior of the car more than 8 and a half feet long to the dash and over 10 feet to the base of the windshield.
If you are planning to place a twin mattress to sleep on it, you should also know that the deck and folded seats are not flat. Due to raised deck housing the traction battery, the folded seats are a few inches lower than the cargo deck.