Good morning starr974,
you can read manga on a 6" ereader without a problem. I've read them on my Kobo Touch.
Here's a video about manga on a Sony PRS-T1:
http://www.youtube.com/watch?v=AguEbIKOb4M
Since I've got my Galaxy Note, I read all my manga and western comics on my phone. With 5.3", the screen is even a bit smaller, but since it reacts way faster than any eInk device, a little zooming from time to time is no problem.
If you're going to buy an ereader especially for manga reading, you should look for a device with decent pdf-features, landscape mode and microSD-slot.
Alternatively, you could also look for a cheap tablet. I'm going to buy an Ainol Novo 7 Aurora after my holidays. I'll use it mainly for reading and some occasional (cough cough) android gaming and I think, it's a comfortable and affordable alternative for comic reading purposes.