Generally if you do plan on printing your comic in the future, you want to make your master file (the one you actively draw on) at the size of what you plan to print at, or proportionally bigger than said size so that you have wiggle room in case you want to change paper sizes. For current printer standards, do you work in 300 dpi for color, or 600 for fine black and white lines.
Keep your master file at those dimensions and just save a shrunken copy for web upload.
690 when at 300 dpi translates to a file that is only 2.3 inches or 5.84 cm when printed at 300 dpi. You need to work in way bigger dimensions if you want to print at full size and not lose clarity.
Whenever I start a piece, my benchmark is always
but if I know dimensions that I have to work around, say Tapas' cover size (960 x 1440 px) that translates to only about 3 inches wide, I will make a proportionately bigger canvas to work on, 4000 x 6000 px (roughly 13 inches wide). Then I can shrink it down for web, and have a nice big original file if I want to use the piece for something else or take it to print.