Screen size is not too much of an issue, since font sizes are quite variable between readers and the bigger sizes are really big (many visually impaired people have turned to these readers and they'll never ever regret it). Thus, do not think too much of screen size.
The device I own, Pocketbook 360, matches your petitions very well: since it operates like a USB stick, the only thing you need to do is download the books, and move them to your device. Its form factor is praiseworthy, and many people in this forum are very happy with its purchase.
However, if time is not a pressing matter for your acquisition, I recommend you to take your time and explore the subforums. Most people are very satisfied with their purchases, others are less, but at any rate reading their opinions will give you a lot of information which will help you. However, the first quick glance should be directed to the
Mighty Matrix, which is a handy way to have a first contact with other devices, aside of the Kindle you've been researching about.