I'm transitioning most of my VMs away from Windows (where possible) to Debian and one of the things that I noticed was that the default resolution of the GDM3 login screen was different from the resolution of the VM once you logged in. Mind you, this is a purely cosmetic difference, but it still bugged me for some reason.
I found that the easiest way to achieve the desired effect - to have the same resolution on the login screen and in the VM when logged in - is actually very simple.
Once everything is set up as desired, run this command:
sudo cp ~/.config/monitors.xml /var/lib/gdm3/.config/
And that's it - now the resolution should be set for all users in the login screen.