The size you want really depends on how you want to print it. It costs more to print custom size books, so going with an industry standard is ideal. You could model it after western comic issues or manga, for example.
In my case, I wanted mine to be the size of a manga. So, when trimmed, it ends up being 5 x 7.5". Though it's not necessary, I work at 600dpi only because I like the artwork to be big enough to support prints or oversized pages if I ever want to do a special edition or something.
But, more than anything, don't forget to factor in your bleed. You need a safe zone, trim line, and bleed factored into your file. If you're working digitally, just make a blank page template and resave it as individual pages to work on.
With the bleed, my pages end up being 6x8.25" at 600dpi.
For web viewing, I actually really like Tapastic's max width. I think it's 940px? It allows you to read comfortably while still seeing all the detail in the artwork, so that's nice.