There are more elegant ways of doing this I am sure, but this should work.

Open the normal.dotm template. When you do this the name at the top of the screen should say "Norma.dotm." Normal Template in Microsoft Word - How to Open

Change the page size. Type any letter of the alphabet. Backspace to delete that (do not use Ctrl-Z or UnDo). Save and close the normal.dotm template.

If you have trouble changing other defaults the same procedure should work. If you want to explore why this is happening, write back in a separate topic about inability to change defaults.