Generally, a bigger screen is better for manga (especially ones with tinier type) but truth be told, I've been able to read them just fine on my jetbook lite. So far I've been able to read Biomeat and Bitter Virgin without issue, and if there's any you want me to test for you I'd be more than happy to toss them on here. The JBL DOES however have a tiny screen so it's not ideal for certain manga, and barely suitable at all for extremely wordy shoujos and seinen.
Since you're planning on using one for manga though, I highly recommend lilman's program Canti, which has been invaluable for turning my manga into pdfs.
https://www.mobileread.com/forums/sho...0&postcount=23