Bebook Club if you are on a budget and have good eyes.
The Bebook Club also displays .cbz-files.
It's a 6" reader, but the Cowboy Bebop series was certainly readable to me.
When you are reading manga in Japanese however, it's a different story.
To read small complex kanji, you will need a big screen.
Simple manga like Yotsuba are not a problem, unless you need the furigana.