No way do I want to haul something around as big as that DX. If I did, I'd read on my iPad. I think you'll find most people seem to prefer a more portable device...even Amazon just released new devices...and none are the size of the DX.
Kindle DX is great for people with bad eye sight, but, personally, I prefer the smaller Nook Touch that is light and fits into my purse (something the DX would never do).
I think that an ideal measure is more than 7 but less tha 9 inch.
I agree that a slightly larger screen (say 7" or 7.5") would be ideal, but I doubt that any ereader producer is going to release this on its own. Currently everyone is using the 6" screen, and almost all are buying from the same manufacturer.
Being the only one using a larger screen would increase their costs and result in a higher product price...
Recently I saw through the web the Xoom Media Edition.
It's a tablet with 8.2 inch.
Perfect screen size!!
8.2 ....8.5 is a very perfect screen size for a much portable e confortably device used in travel on public transport, airplane, etc. and in a landscape mode you can read without much eyes stress.
Both for a tablet than for an ereder.