If you are producing a standard-sized comic (manga-sized as well) then your pages should have a ratio of 2 parts to 3 parts. Using this ratio you can scale the dimensions of your page and still keep the layout in spec.
For example — for a standard sized comic the canvas size of the files you send to us is 7 inches wide by 10.5 inches high. That’s a 2 parts to 3 parts ratio.
If you divide the width (7 inches) by 2 you get 3.5. If you then multiply 3.5 by 3 and you get the height — 10.5. See? 2 to 3 ratio.
So if you want to draw your comic on larger sheets of artboard and be sure that you’ve got the right dimensions just follow that ratio. If you want to draw at 10 inches wide then just divide 10 by 2 (=5) and then multiply 5 x 3 and you get the proper height — 15 inches.