MacBook: Cannot select paper sizes other than Letter

On Word for Mac, set the paper size through Format > Document > Page Setup instead of Layout > Size.

Steps:

1. Open a blank document.
2. On the macOS menu bar, select Format > Document.
3. In the Document dialog, select Page Setup.
4. Set Paper Size to the size needed, such as A4.
5. Select OK.

If the goal is to make that size the default for new documents:

1. After setting the paper size, stay in the Document dialog.
2. Select Default....
3. When prompted to change the default settings for page setup, select Yes.

If only US Letter appears in the ribbon’s Layout > Size list, using Page Setup is the supported workaround shown for Word for Mac.

• Word Mac - When creating a new document it defaults to letter instead of A4 - Microsoft Q&A
• Print a document in Word for Mac