I would say at least 7", 8" or 9" would be better. 10" and above are awesome for comic reading, but their weight are too heavy and size are too large for portable usage.
I'm using the Kobo Libra H2O which is 7", it's okay for comic but I wish I had bought the Kobo Forma 8" at the time.
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I'm using the Kobo Libra H2O which is 7", it's okay for comic but I wish I had bought the Kobo Forma 8" at the time.