Comic book pages here in the United States are traditionally drawn on 11″ x 17″ boards. There are usually blue lines printed on these boards to help the artist determine where the live area, trim area, and bleed area lines are on a given comic book page which are super important things to keep in mind when going to print. (Read up on page sizes and live, trim, and bleed. I swear you’ll thank me.) Now, if you’re creating your own webcomic, you don’t have to adhere to any industry standard. There are plenty of creators out there who draw on US Letter sized paper (8.5″ x 11″, most often because it’s cheap and common), manga sized boards (A4, 8.3″ x 11.7″), comic strip art boards (5.5″ x 17″), and so on. You can draw your comics on bar napkins for all I care, but however you create your artwork, it must be digitized if you want to make it a webcomic.