There are different requirements for the page size in different areas. For example, Letter size is most popular in North America while A4 size is popular in some Asian countries. The default page size in North America is Letter size, but you can always change it to a different size. Please see below for details:
Step 1: Click the "Layout" tab from the ribbon;
Step 2: Click "Size" and select the size you want to use from the drop-down list;
Step 3: If you cannot find the page size that fit your document, you can click "More Paper Size" at the bottom to open "Page Setup" window. In the new window, change the page size according to your document.