Those are good questions, Tony.
When I first started doing comics at HIP Comix, Timo guided me through the process. One of the things he told me was that I should make my covers bigger, so I just looked at the covers for his comics and made mine the same size (1250X1750). He never told me to make my pages bigger, so I've kind of stuck with what I was doing. My pages in Ring of Domination were all square (800X800), but I decided I could do more and make things look more exciting if I used rectangles (either vertical or horizontal). I basically chose 800 and 1000 as my dimensions because that was about as big a picture as I can fit on my laptop screen without having to zoom out to see the overall picture. E.g., when I read Uroboros' comics, I read the dialogue at full size, but usually zoom out to see the whole picture.
I occasionally do a "splash page" that is bigger than my usual page size. The last page of Victory City #26 is like that, as is the last page of Victory City #9. I accidently made the first page of Adventures of Ishtar #4 bigger than usual, and didn't realize until months after I posted it

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If people would like me to do bigger pages, I can. It would mean longer rendering times, but if enough people want it I could do that. I have basically no formal training in art or computer graphics, so if I do anything that makes my comics less accessible, please let me know

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