Unless you create this as the default page size, I'm not sure what you can do besides creating a macro to do it, which is a very simple process and will allow you to instantly change your page size. If your Developer tab is not showing, go to Word Options and check the relevant box on the Popular page. go back into your document, select the Developer tab, and select Record a Macro. In the pop up dialogue, give it a name, like Size, and add it to a menu or assign it a keyboard shortcut. (You can look these aspects up for further help.) Once you have done so, your cursor will become a little "tape recorder", and you simply perform the actions you need: Page Layout/Size/More Sizes/Custom size. Enter your dimensions, choose either whole document or This section, and press OK. Back to the Developer tab and click Stop Recording.