Accurate editing view

Can you make the editor show an accurate depiction of what the live site looks like? It’s very frustrating to get cover photos and everything looking great and then going to preview and it looks totally different. Dimensions are different on iPad versus computer versus Apple versus android and I have no idea if my stuff even looks good. This seems to be a basic function on any website builder. I would love the option of a header photo. Give us dimentions for desktop and mobile and we can create photos that we know we can see on both devices.