There are a few ways to set a default label size for your Label Printer. 

Normally, you can use "Last Used Settings" shown below as a preset to print. This will remember the setting that was previously used. 

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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"

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Click on "+" to add a new size. 

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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.