Are we talking size or resolution?
Size I just limit it to the webtoon rules since I post there, I think it's 800x1280 or something like that.
I do a long format comic, and keep in mind I am pretty new to this and probably have no idea what I'm doing, but so far it has been working for me, but I typically draw pictures at 350dpi, and then copy paste them into a super long strip that is 72dpi, and arrange them into the panels. Then cut that sucker up with image magick(a program... probably using the same code that these comic websites use to piece the art back together).
I tested out doing the actual comic at a higher resolution, but since I upload on webtoon and webtoon makes you convert to jpg, the quality declines regardless. And working at a lower resolution on a large strip(I'm talking 800x25,000 long) is just faster and easier on my machine. I could probably cut the comic up more when I work on it, but i like having as much as I can on one picture because it helps me when I finish it up.