Somewhere in your Printing Preferences you should have a Paper Tab and then Paper Options. For instance if I click on Paper Options/Original Size I have A4 set as default. If I then click on A4 I get a drop down list and at the bottom I have Edit which then gives me a Custom Paper List. I am able to select any size either in millimeters or inches. Take a look in your settings - you should have something similar.
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