I love printing, I could talk about it all day!
Work backwards: what would you like your final book to look like? I enjoy the slightly taller than wider books on my shelf and got my ratio from measuring up the Scott Pilgrim books and looking at what people use in industry. Ratio is more important that size, it determines what your finished book will be like. If you want to be able to print it out yourself and staple it up like a zine, an A4 ratio is the best bet; it translates neatly down to A5 for an A5 zine neatly. Advantages: you can print it and bind it yourself, which is really cool. If you want it to look like manga, measure a book you have and work out that ratio.
I personally work in 11x17" for my comic pages, which is the same size bristol board as used by pros that work traditionally (plus two inches round the edges for me to scrawl that gets trimmed before upload, so my "new doc" preset on CSP is 13x17). It lets me work big enough that I can get detailed, but not so detailed that I lose information on the web or in print if I choose to print. It also means you don't have to zoom to 400% to work in detail (my rule is never zoom more then 150% or I'll lose all that information when publishing). If you want it printed to look like a traditional comic book that you might see printed up in a shop, that's a good size. Compared to the A4 ratio, it's slightly taller. If you like the A4 ratio, work bigger than A4 (A3 if your computer can handle it, or size up the canvas and keep the same ratio). Your lines will be crisper and smoother when working bigger and you don't have to do that 400% zoom thing where it's all pixelly and gross. Basically work bigger than you want to print.
You can also have a look at this link, which is all about page ratios. It's a great resource.
If you're getting your books professionally printed, you can have them in whatever size you very well please. Non standard sizes will be more expensive but it can be done.
Resolution, 300dpi for colour, mimimum. I work in 350dpi (mostly because I can). 600dpi or higher for black and white; traditional manga artists scan at >2400dpi, and work huge it's how they get those amazing fine lines.
Publishing on tapas/webtoons you're best off doing those big tall strips rather than pages, because they're optimised for scrolling (tall and skinny with a straight downwards flow, while a normal page you go more left and right in a zigzag). Traditional comic pages look really, really bad on here and webtoons just because of the resolution, so keep that in mind as well if you're doing delicate linework! Eg. my 11x17 pages are 24% of their usual size upon upload. It's painful.
Anyway I love printing/resolution/bookmaking talk if you wanna talk about it/have more questions feel free to dm me! Goes for anyone reading this too, it's a super confusing topic but I love to chat about it, happy to chat to fellow interested folks.
