Just speculating because I'm looking for distraction...
At first I downloaded the PDF from the FCC and wondered if there was any chance that the label on the A4 PDF page was fitted at 100% - which would make it a 7.8" screen.
However, when I printed out the page I realised that the header (buttons and "Settings") is too large when compared to the same display on my Aura One. So then I tried resizing the page display until I got the header into proportions that seemed reasonably similar to the display on my Aura One, and that came up with a 6" screen.
So if anyone's making book, I'll put my money on 6", 300 dpi.
(I was going to keep this guess to myself - but where's the fun in that?
)