tapas provides this quick guide on the publishing page
but the limit on width is 940px (like it's shown here)
you can do any page layout in terms of vertical scroll (or webtoons style as it's sometimes referred to) or traditional layout like physical comic books
also if your canvas happens to exceed 4000px or exceeds the 2mb limit you can always slice the image
this thread points to one online but there's also tons of others or (if you have the software) you can use photoshop's slice tool
