6" ain't big enough.
I don't know how everybody settled on 6", but it's pathetically small. Not even the size of a mmpb, and a book opens up to two pages, so it's not even half the printable size of one book page turn.
DX or tablet size is too big for portability.
With a touch screen UI that everyone is moving to, we should be able to get something the size of a mmbp with the same "printable" screen area, around 7 to 7.5".