There's a new Clip Studio feature that allows you to splice and convert your pages into Webtoon and Tapas parameters.
I make my width 2000, so the specify width currently is at 100%. Here, I need to change it to the width parameter of Webtoons (800) or Tapas (940). That will scale it down.
For the divide vertically, the X < px has to be smaller than the height you currently have. I put mine down to 1000, and then is ends up showing how many pieces it will give out.