I don't really see the room for demand beyond 8 inches ever growing, largely because most bigger books are just far more complicated to transform to a digital format (e.g. textbooks with multiple sections on each page and tactically positioned images)

...but, Kobo in the last two years seem to have done pretty well with h2o and aura one models. I'd be very interested in seeing how the h2o in particular performs compared to their smaller models; 6.8 inches seems like a pretty ideal compromise between size and portability for me that 6 inches does not.