So regardless of what your paper size is, the max width they allow is 800 pixels. So if you scan your work it at any decent dpi you're going to have to scale it down regardless.
If you mean more like what ratio of drawing to use, that's a little trickier. They prefer the vertical scroll format which pretty much assumes only 1 or two panels in a row, so you might only be able to fit 2-3 panels on a page. There's nothing wrong with doing more traditional pages, but a lot of readers on webtoons don't like that xD