The fact that they are going with 6 inches, plus the emphasis on the ability to stream back to the controller keeps making me go back to the idea of a 3d enabled display on the controller.
I mean, why bother with such a big touch screen otherwise? 5 inches would be fine... Heck, 3 inches would be fine if gameplay was the main reason it was there.
But if you intended for people to use the controller as a main way to view the games for the system because you were going to use 3d tech in your screen, then you would be likely to use an especially big screen, and you would spare no expense to make sure that the streaming tech for the wireless video was top notch.
I know it would cannibalize the 3ds, but honestly, they have the tech, and it can't be properly implemented in televisions for a reasonable price at the moment.