Click the "Print Label" button -> At the top of the popup, select the 4x6'' label format.
Pls check the links for the details:
https://support.pirateship.com/en/articles/2610080-windows-how-to-print-with-a-4x6-label-printer
https://support.pirateship.com/en/articles/2607130-mac-how-to-print-with-a-4x6-label-printer