Basically what he said ^^^^ Tapastic supports mobile comics so the best method is to go for as high as you want, but keep width in mind.
HOWEVER, that doesn't mean you have to restrict yourself to 940 px when working. When I do my own visual comics, let's say for Time Gate: Reaper, all my pages are 8.5 x 14 inches, which is just a bit longer than the standard paper size, and I work at 350 dpi. With Uzuki, I do my pages horizontal first and then reformat them for vertical, and they end up being on a 3 x 9 inch canvas for black and white and a 3 x 12.149 inch canvas for the color ones.
Then once my pages are done, I throw 'em in Photoshop and adjust the page size to 940, not touching the height (because this adjusts itself to suit the change to 940). This means that I still retain the same quality despite the size change.
So definitely try and work in 300, 350, or 600 dpi (don't do the default 72 dpi for programs like Photoshop, it's subpar and gross to work with). Keep your sizes big and then resize after so that you don't end up sacrificing the quality while working (: As long as your width is 940 px when you submit it to Tapastic, you're all good (as for the size limit, it's very VERY difficult to break that limit after resizing it, even with huge beautiful pages with stuff like Fisheye Placebo and Ghost Blade. If you're worried about overgoing the size limit, just cut your pages into multiple pages and make them separate files to upload).
Hope that helps!
