There are a few ways to set a default label size for your Prafia printer.
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, follow these steps to set the default size for your operating system:
In the print prompt window, under Paper Size, click on Manage Custom Sizes.
Click on the + button to add a new size.
Create a new size with dimensions 4" x 6" or 100 mm x 150 mm. You can double-click on the left to rename this setting to something like 4x6 or 100x150.
Then go to System Preferences → Printers & Scanners → Select Prafia Printer and set the default size to 4x6 or 100 mm x 150 mm.