A family of Microsoft word processing software products for creating web, email, and print documents.
While the others bicker about who said what [both being wrong about what the other replied, BTW] let's shift the focus back to you & your issue :-) First, please clarify that this is exactly what you're doing... Defining the Default Paper Size should require nothing more than these 6 steps:
- Go to Format> Document
- Click the Page Setup button
- [The defaults cannot be changed by going to File> Page Setup... any changes made there affects only the current document.]
- Select the preferred Paper Size
- Click OK
- Click the Default button
- Confirm updating the Normal template
If that isn't working for you there is something interfering with the process. To start determining what that is please go to Word> About Microsoft Word in the main menu to determine the present Version & the type of License for your Office software. Include that in your reply as wall as indicating the flavor of macOS you're running it on.