6in satisfying? Not to me.
I find 5in more satisfying because of portability and lightness; it is simply less stressful to hold. Part of it is ergonomics (I really like the design of my Pocketbook 360; both the hardware and software) but part of it is that I find 6in to be something of a "tweener" size.
Given the lack of alternate products, I can't say for sure but I suspect a 7-8 in screen in a 16x10 aspect ratio would result in a better mid-range product than the 6in products out there.
For bigger screens I think I'll have to wait for plastic-substrate color screens before I find anything satisfying. The iPad is definitely too heavy and even the KDXG looks too heavy for extended sessions. I have some hope for the PB90x but even there...
In the end we'll have to make the most of what is offered because it doesn't look like the vendors have much interest in experimenting with alternate screen sizes/aspect ratios.