To add a page layout, right click on the "Layouts" and select "Add page layout". To rename a page layout, right click on the layout and select "Rename".
A document layout has the following regions:
You can edit the page layout by either double clicking on it in the pagination window or selecting an option from the menu: Page Layout →Layouts
In the page layout properties window, you can define both the page size in general (in the Page properties) and the size of the different regions.
Important: The body region (where the main content of the report is added) does not have a size (the page has it), but it has margins and padding. The margin is the total margin from the page size to the body content. So if you for example would like to have a header that is 3 cm high (Extent) you need to set both the extent of the header and make sure that the body has a top margin of at least 3 cm. Otherwise, the content of the body and header would overlap. If you check the precedence option for the header or footer it will fill the total width (precedence over the left and right regions):