Most people are finding the iRex DR1000's 10.2" diagonal screen big enough in portrait mode after croping the margins of letter/A4 PDFs. The alternative is to switch to landscape mode and display half a page at a time at full size. The Plastic Logic 10.7" screen will be a bit better and the PVI 9.7" screen (in several devices this summer) a bit worse. If Amazon produces a big screen device it will probably use the 9.7" screen, like the Netronix EB-900. See the
E-book Reader Matrix for the DR1000S and the EB-900.
There are supposed to be 5" and 8" screens from PVI too, see
EInk: High Resolution Displays. There will probably be a device from Jinke (Hanlin) using the 5" screen, but I have not heard of any new devices with an 8" screen. If this used a touch screen it could be the same size as the current Kindle 2 but with an on-screen keyboard. Based on the iLiad, an on-screen keyboard is
much easier to use on an 8" screen than a smaller screen. If the screen were robust enough this might make a good form factor. Perhaps I just described the Kindle 3, with (as reported by various sources last year) the 9.7" device as a separate "professional" model for students and others who need PDF support. I would make it touch screen too, but Amazon really likes actual keyboards.