It depends on how you want your finalized comic to look and if you want it to be digital-only or printed later on.
If you're doing digital-only, it's probably best to do the vertical scrolling style. For Tapas, it's 940 x 4000 (but a smaller height is better for smaller image size which means faster loading). There's no height restriction because you can make it as long as needed and then slice into multiple images.
If you're interested in printing at some point, then I would recommend looking into standard sizes. Like most US manga is 5" x 7.5". So then make the canvas that size with a 0.125" bleed around it (total size will be 5.25" x 7.75"), 300DPI if working in color, 600 DPI if working in black and white.
