There are a few ways to set a default label size for your Phomemo 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 for any reason, please follow these steps to set the default size for OS
In the Print Prompt Window, under Paper Size, click on "Manage Custom Sizes"
Click on "+" to add a new size.
Create a new size with 4" x 6" dimensions. You may double-click on the left to rename this page or something similar.
Please go to System Preferences -> Printers & Scanners -> Select Label
Printer and set the default size.