1. Open the Word document, enter the page layout tab, and click the "margins" option of the document.

2. In the Page Margin and Paper Size dialog box that opens, select the Custom Page Margin tab, select Mosaic Binding in the drop-down menu, and then manually enter the width of the spine. Then in the "up" and "down" positions, the corresponding height values are also manually entered.

3. After confirming the set spine width and height, click OK to finish the setting.