I paint my comics much bigger than screen resolution, usually at about 300 dpi and about 8.5 inches in width. When I save for web, it shrinks it down to the dpi and width required for Webtoons/Tapas. Makes it nice and clear.
For really long episodes, I thumbnail on a long canvas--but I do the painting and layers for each panel on their own seperate files. Then, I copy those files (as one layer per image) and paste it into the final long canvas.