Crystalbear: Manga are typically in a ratio of 1:sqrt(2), the standard paper size ratio in Europe and Asia. Manga magazines are Japanese B5-size (182x257 mm), while the books are B6J (half that, 128x182 mm) or some similar size (they vary between publishers, though B6J is the closest there is to a standard size). American comic books are typically 17x26 cm.
Ratios are pretty flexible, look into the many standards rather than looking for just one. Some comics might even work best at a non-standard size. Instead of thinking in terms of screen size when doing your art, think in print sizes. If you are to print this comic, what options are available to you? Even if you don't want to print it, it might be nice to pick one of the standard sizes and work at print resolution just in case (300~600dpi). Then, just shrink down to your screen size.
As for screen size: Single pages should mostly fit on a typical screen. About 1000px in height is okay, as the scrolling will be minimal. Larger than 1000px is pushing it. Do not make your pages wider than the typical screen, so avoid pages wider than 1200px (most screens are between 1280 and 1900 wide these days, but not all of that room is available for you).
If your single pages are sized well, then a double-pager should also fit nicely. The heigh will be the same, and if the width is good, then there should be no problem with horizontal scrolling. (or the scrolling will be minimal). If you really want to be good to your users, then make your spread, then size it down so that the width is less than 1200px. The height will probably be less than you'd like, but if you've planned things out well and chose a legible font, the slightly smaller size will not be an issue.