windows 10 – How to set default paper size in Edge browser?

We are using Microsoft Edge browser (Edge Chromium, Version 91.0.864.41 (Official build) (64-bit)) to print receipts from an online ERP system.

We want the printer (an OKI 411dn) to print A6 by default but we can’t get it to work. The printer natively supports A6, but the edge dialogue does not show A6 in the print dialogue dropdown.

Now, according to https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#printingpapersizedefault you can set a default paper size, using the PrintingPaperSizeDefault registry key. So we tried to a custom value that fits the A6 size, like so:

via regedit

Viewing that value in Edge looks like this:

via edge://policy/

However, when we check Print dialogue in Edge browser, we cannot see the custom paper size anywhere:

print dialogue Microsoft Edge

We have restarted the machine, installed latest drivers, tried ‘Print to PDF’ etc.
Also checking Print Server Properties in Windows 10 gives us the following (notice A6 is there plus a bunch of other forms). What are we doing wrong??:

Print Server Forms