I'm not the best person to answer this, since I haven't had any of my own work printed yet, but what size you make your canvas to start with obviously depends on what size you'd ultimately like it printed. If you want it printed in a manga format, you're going to be best served by having your canvas size the same ratio as a printed manga page (unless I'm very mistaken.)
Personally, I use a western page format, and I know the page size that professionals who still draw by hand use is 11 x 17 inches, so that's what I use -- the printed format is that ratio, just smaller.