There is also the smaller
Trekstor Pyrus Mini, but like the Pocketbook 360 its discontinued, and probably difficult to find.
I have the Sony 350 and recently bought the Kobo Mini as I hoped it would be an improvement on the PRS350 just by virtue of it being a little newer. The touch screen is a lot more responsive & battery doesn't drain so much when not being used, and its lovely and small and light. But I hate the Kobo case compared to the far slimmer Sony case, and more importantly, I can't bring myself to use it a lot because the screen is even darker on the Mini than my old PRS350 (though the fact my Mini is white might be making it seem worse?), and I find that really off-putting.
Sadly, there doesn't seem to be much of a market for smaller eink readers. :-(