Normally, you can use "Last Used Settings" shown below as a preset to print. This will remember the setting that was previously used. If this doesn't work for any reason, please follow these steps to set the default size for OS.
In the Print Prompt Window, click on 【Manage Custom Sizes】 under Paper Size,
Click on "+" to add a new paper size.
Create a new size with 4×6 dimensions. You may double-click to rename this the paper size.
Please go to System Preferences -> Printers Scanners -> Select HotLabel Printer and set the default size to 4x6.
Go to printer settings, and you will find the default paper size of the printer is set to 4*6 inches now