In the PageTab click on Size in the Page section. This opens a dropdown menu of standard page sizes. This will change the size of the current page. You can also enter your own custom setting or change the default you can do so by clicking on Page Size Setup.
In the window that opens you will first need to decide which pages need to be changed. You can choose from the options available in the Page Range section.
The Fixed section will let you pick between the already existing page sizes. Choose from the dropdown.
In the Custom section you will be able to choose a unit of measurement. Once you have done so, enter the dimensions for the length and the width of the page.
Choose the Make Default and the new page size will be used when new documents are created. When you are ready, click on Ok and the changes will be made.