Large screen is a must, but you don't have to get a monster like a Que or a Skiff. I've seen people happy with reading in 6 inches but I am not convinced. Probably an iRex device (iLiad, 800 or 1000) would fit your needs. You can wait some months though, since cheaper 9 inch readers (Hanlin and Pocketbook) are due to appear this summer.
However, you'd do a good thing if you started converting your manga. I'm afraid Cbr and Cbz are still a bit out of firmware's possibilities. I'd recommend you a certain little program for
converting them to ePub.