How do I change paper size of exported .docx(outlined) file?

When I read a file exported as .docx(outlined) by Microsoft Word for Mac, paper size is always set to LETTER. But I want to change it to A4 not by using Word settings, but to set OmniOutliner default to A4. I am a user of OmniOutliner 4.1.2. How can I do that?