I know this is pretty late, but I just wanted to thank you macropod and gmayor for your help! It was driving me crazy that I couldn't figure it out.

Between your two comments, I was able to figure out that I had to A). create a custom defualt print size (that's in the dropdown menu with A4, US Letter, US Legal, etc.) with no margins and B). select the "any printer" dropdown in order to be able to select my custom size. Worked like a charm! Thanks again!