There are few ways to set a default label size for your Rollo printer.
Normally, you can use "Last Used Settings" shown below as a preset to print. This will remember the setting previously used.
If this doesn't work for you for any reason, please follow these steps to set the default size for OS
In the Print Prompt Window, under Paper Size, please click on "Manage Custom Sizes"
Click on "+" to add a new size.
Create a new size with 4 in by 6 in dimensions. You may double-click on the left to rename this page to 4x6 or something similar.
Please go to System Preferences -> Printers & Scanners -> Select Rollo Printer and set the default size to be 4x6.
For any questions, or should you want any assistance with this, please let me know.