how to set a default size for a blank page

You should use the "NewDocument" plug-in which is part of my (free) abracadabraTools:

https://www.abracadabrapdf.net/utilitaires/utilities-in-english/abracadabratools_en/

Capture_2601181726.png

Not possible, but you can use a script to add your own menu item that will create a file with a page of the size you want.

@pierret18811376 Acrobat does not currently have a preference setting that allows you to change the default size for a new "Blank Page" (which defaults to US Letter, 215.9mm x 279.4mm). This is a known limitation that many users find frustrating. Granted, on my document, I work in inches, so, it rounds up as 8.5" x 11" perfectly! 

If you have already created a page and it is the wrong size, you can fix it instantly without starting over. Go to Tools - Organize Pages. In the secondary toolbar, select More - Set Page Boxes. Under "Change Page Size," select Fixed Sizes and choose A4. Also select "Page Range" is set to All if you have multiple pages.

Or you can also use the Preflight tool as well