Not sure what else to suggest here. What graphics hardware do you have? For NVIDIA or AMD hardware it may be possible to switch to using a proprietary driver.
If you can manage to open a terminal window, you may be able to use xrandr to adjust the display size using something like
xrandr --output HDMI1 --transform 1.05,0,-35,0,1.05,-19,0,0,1
*This assumes the output device name is HDMI1 (you’ll need to get that from ‘xrandr’ first if not sure).
Reference:
https://newagesoldier.com/linux-hdmi-resize-screen-overscan-fix-ubuntu/
If that works, you may then be able to configure the required display settings at start-up.