Adding a custom paper size in Darktable

I’m running Darktable on Ubuntu, and I have added a custom paper size in my main printer dialog. However, this does not show up in Darktable’s print module. I’m trying to print to a 3×5 index card but there’s nothing this small in the options. Is there any way for me to create my own paper size?

Are you using snap?

No, I’m using the OBS repo for Ubuntu.

OK, I just switched to the appimage for other reasons. Does that get me any closer to defining my own paper sizes?