Page Layout menu option in Word 2013 and Word 2010 provides users with option for changing the page size. In case if you want to change the page size from the standard A4 size to A3 then you can use this feature.
From the Home menu, navigate to Page Layout menu.
In the Page Layout menu, navigate to Page Setup section.
To change the Page Size for the Word document, click the drop down arrow below the Size menu option and select your preferred size for the Word document.
If you want to change the page size only during the printing of the document, then you can use the Print option for this purpose.
Click the File menu and then the Print menu. Then navigate to the Settings section on the right hand side.
This has an option for setting the page size for a document as shown below.