Here's a neat article on Comic Book Page Aspect Ratios & Templates
"Webcomic? Graphic Novel [long form], Ero album, American pamphlet? Will your book be digital first or pint? Standard American comics typically are printed 6.63″ x 10.24″ (16.84 x 26.01 cm). European comics typically come hard bound, and 8.4″ x 11.6″ in (21×30 cm). Trade Paperback graphic novels vary widely, and there are a few standard sizes for Manga, Zines too. This is something we MUST resolve before we start drawing. So give it some thought."