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.
What is Amazon Go and will it come to the UK? The store without checkouts or queues