How do you set the default page size to US sizes?

The default page size on MuseScore seems to be set to Metric paper size, A4. How do I reset it so that the default is "Letter / ANSI A", which is the standard size in the USA?

It only reverts to the default (A4) on the first document I create after opening MuseScore. When I change the first document to "Letter" size paper, the subsequent documents use that setting, but when I Quit and reopen, I have to again set the paper size.