To set the default page size in Word for Mac:
- Create a new blank document.
- Choose Format>Document from the macOS menu bar.
- In the Document dialog, click on the Page Setup button.
- Set the Paper Size to A4, then click on OK.
- Back in the Document dialog, click on the Default... button.
- When Word asks Do you want to change the default settings for page setup?, click on Yes.
- Test by creating a new blank document.