Welcome to the Make community!
You can change the page size using the documents.batchUpdate endpoint.
However, I think you should be able to change the default, by creating a new document, then go to File > Page Settings
Change to A4, and click “Set as default”.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!