- Click the Page Layout tab
- Press the little arrow at the bottom right of the Page Setup area.
- Click on the Paper tab on the dialog box that pops up.
- Enter in your desired paper size into the Width and Height boxes.
Note that Microsoft Word limits either dimension of a page to around 56cm so I'm afraid you will not be able to get four times the area of an A3 page, but you will be able to get about half that.