Inside each page you will have the ability to insert an artboard. You can think of an artboard as the canvas to your web or mobile design. When creating an artboard you will be given several templates that can jumpstart your design, such as Apple devices, material design, responsive web, paper sizes, and, of course, custom. Regardless of the template that you select, you can always manipulate the sizing of it based on your project. Generally, I would proceed with the recommendations from Sketch as they are fairly accurate to start off, but if your project calls for a different dimension this is the opportunity to change it. Whatever you do, ensure you are consistent with the width of your artboards for your project. These are the actual pages of your design. Organize your artboards strategically within the canvas and be sure to create duplicates of your artboards for the various pages and states.