If your goal is to print your comic using traditional channels, disregard this article. Publishing for print is completely different than digital and should be respected as such. However, if your goal is to become a successful digital comic artist, then proceed.
Think Mobile Before Anything Else
In countries like Korea, web comics are all the rage (Koreans call them "webtoons"). This is because comic publishers started thinking mobile-first when it came to formatting. We don't want to dictate with an iron fist the format for your title, but we do highly (HIGHLY) recommend thinking about the mobile user first.
Most people simply want to scroll down and read their content. This means a web comic should stack for optimal experience. If you look at the most popular webtoons out there, you will see they are read vertically and not horizontally left-to-right like traditional comics.
You can really take advantage and have fun with this format. Designing your issue layout can be just as important as the comic art and writing itself. Try having one long vertical frame, with text and visuals, that unfolds with scrolling. The creative possibilities are there! This does mean you should avoid double-page spreads that look great in print, but are sub-optimal for mobile.
Think Widescreen Video
If you need a wider canvass to communicate the story, then keep frames within a widescreen video aspect ratio. The reason for this is that almost every phone follows these dimensions to accommodate video. This means if your comic is 1080 pixels wide (like Instagram), try to keep the height to 608 pixels. At the least, make sure the important information is within that frame so users don't have to pinch or scroll up and down to get what they need.
Think Instagram and Facebook
Another reason we chose the 1080 pixel width format is the ability to promote across multiple social media channels. By far, Instagram and Facebook will be your best sources for marketing. And if your content can fit into Instagram's sizes, then you will have no problem with Facebook, since they are the same company. It simply helps if you can format your art once to look good on Tabulit and social media.