Hi, not sure if you figured out how to change resolution of login screen or not but I recently got this working myself so this may help you or others.
I found these 2 links as my starting point:
See post titled "Probable Cause"
See 6th answer that talks about editing/creating a file in the /usr/share/X11/xorg.conf.d directory
I wanted a lower resolution for the monitor I have connected to my pi 4B instead of 1920x1080 it chooses as default - was creating flickering/buzzing sometimes.
Anyway, I ran xrandr --q to see what modes were already on mine:
Screen 0: minimum 320 x 200, current 1280 x 720, maximum 7680 x 7680
HDMI-1 connected primary 1280x720+0+0 (normal left inverted right x axis y axis) 300mm x 260mm
1280x720 60.00*+ 59.94
1920x1080 60.00 + 50.00 50.00
1680x1050 74.89 59.88
1400x1050 74.87 59.95
...
[Note that xrandr reports that my preferred and current setting is 1280x720 because I already changed it, but it was showing 1920x1080 as the preferred when I first ran it]
It was defaulting to 1920x1080 but I wanted 1280x720 so I created a file I called 55-preferred-login-res.conf and put it in the directory /usr/share/X11/xorg.conf.d like the 2nd example I included above had. The contents of that file is:
Section "Monitor"
Identifier "HDMI-1"
Option "PreferredMode" "1280x720"
EndSection
$ sudo cp 55-preferred-login-res.conf /usr/share/X11/xorg.conf.d
After shutting down my raspberry pi and starting up again, it starts at 1280x720 resolution, so that worked for me.
Peace.