Sure, I'd be happy to help you add a missing paper size in Excel!

Here are the steps to follow:

1. Open Excel and click on the "Page Layout" tab in the ribbon at the top of the screen.
2. Click on the "Size" button in the "Page Setup" section of the ribbon.
3. At the bottom of the drop-down menu, click on "More Paper Sizes."
4. In the "Page Setup" dialog box that appears, click on the "Paper" tab.
5. Click on the "Custom Size" button.
6. In the "Page Setup" dialog box, enter the width and height of the paper size you want to add in the "Width" and "Height" fields.
7. Give your custom paper size a name in the "Name" field.
8. Click "OK" to save your new paper size.

Your new paper size should now be available in the "Size" drop-down menu in the "Page Setup" section of the "Page Layout" tab.