I don’t think this is possible because the page definitions come from the printer driver.
What you can do instead is to prepare a template with all your settings, notably a custom page style with a page size offered by you printer.
The printer-known page size will try to outsmart the printer driver by declaring to use this sheet size but the page style (and template general layout) will add margins so that the printed area is exactly the one of your custom format.
You will need trial and error to determine the correct parameters (mostly margins) because it depends on how non-standard sheets are mechanically guided into the printer. Some trays may have only a single slider aligning paper to left or right. Others have mechanically linked sliders which center sheets.
Then when creating your labels, you open a new document from your template and everything is already in place.
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