WORD page setup options. A4 NOT letter PLEASE!!!

How can I stop WORD defaulting to “letter” in page setup? I want to use A4 from tray 2 but it will insist on reverting to LETTER – ARGGGGH!!
It means that every time I send a document to the printer I have to go into Page setup and change it manually or feed a page into the tray before the printer will accept it.