Re: er, why not the iPad
5.7 inches isn't very far away from the iPad Mini size. a 6 inch phone is practically like holding a tablet to the side of your head.
i'll be very surprised if this actually comes to market as a phone.
it could easily be the screen they are testing a remote control for the AppleTV or something.
Exploring doesn't mean they're going into large scale production, just means someone at Apple has presumably ordered a few of these for R&D