A ComfyPage web page is divided into the following areas.
The header, footer, left and right margins appear on every page. This means that if, for example, you update the header it will be updated across the whole site.
The size of each area depends on the contents of each area and your border width settings which are available in the Style Editor. Generally they will resize to fit whatever you put in them. Any of the areas can be left empty and will automatically be made narrower unless you have specified that it should stay a particular width in the Style Editor.
So sometimes the left and right margins can change size from page to page. If this is an issue see the FAQ's Question #6 for a possible solution.