Before CSP updated with the webtoon splicing feature, I would put it through croppy -- which would resize, splice, and convert the pages. Now I just use the CSP feature, and it does what it needs to do based on the parameters I give it.
And if it doesn't fit in terms of MB, I just throw everything through tiny.png or tiny.jpg. I've got three comics on Webtoons with 60+ panels each update -- and I made them fit. Haven't tried them with Tapas yet, but with the methods I have, it shouldn't be a struggle.
Important to note: the parameters listed for both sites are the max. I'm pretty sure as long as it's equal to or less than those parameters, it will take. So I can also imagine some people working with, say 600 px or 700 px as the width.