Two things I've considered recently about horizontal screen width while shopping and updating my set of readers and tablets:

1. How wide a line of text can I scan rapidly and wrap lines efficiently without losing my place. There's a point of diminishing returns where a single column of text on a screen gets too narrow, or too wide, for me to read efficiently. Too narrow, and I'm wrapping so often it becomes tiresome. Too wide, I begin to lose my place when I wrap lines and it breaks the flow of reading. I can compensate somewhat by adjusting other details such as font size and vertical space between lines. Still, a ballpark line width of maybe 4-5 inches is about what I can consume efficiently at normal reading speed.
2. How wide a line of text can I read clearly with minimal head movement. The progressive lenses in my glasses have a "sweet spot" that is a fairly narrow band for reading. If I get lines of text too wide, then at normal reading distances and font sizes, I begin having to turn my head with each line to keep the sweet spot on the text. For some reason if I have too much head movement, it becomes a drag on reading comfort and a barrier to getting into the 'zone' where I'm fully engaged and reading fast. For folks with younger and better eyes, obviously this is all moot.

That's why, for a device used mainly for reading books, I incline toward screens in the 7-8" diagonal range with a usable width of 4-5". I wonder if a device of this form factor ends up being optimal for a good-sized set of other readers as well. If so, you wouldn't know it from today's line-up of available tablets and readers, there are relatively few in that 8" range.