Hi there Wolf!
I have a couple of articles here that I believe will help you out with your question. First, here is an article with some information on custom paper sizes:
Note that in that article, it states:
Custom paper sizes aren’t available for some printers.
The reason for that is because this is controlled by the driver for the individual printer. You will want to make sure you have the latest version of the printer driver installed on your computer. More information on this can be found in the following article:
It might also be helpful to reset your printing system to see if that will resolve the issue. More information on this process can be found here:
Take care, and thanks for visiting the Apple Support Communities.