How to change the login screen resolution in Ubuntu 18.04

I need a way to change the boot-up and login screen resolution to 1920 x 1080 resolution. I have a 4K display on my laptop and can't read the text in the grub bootloader screen properly as they aren't scaled. Even the login screen have some qirkiness like it missed scaling the mouse cursor.

I figure I could solve this if I can just lower the boot-up resolution and login screen, but I don't know what's the best way to about doing this. In the past, I have changed the boot-up screen resolution via kernel boot command parameters and muck about with gnome gdm. But if I don't do things properly, the changes may be overwritten during regular Ubuntu update.

Please provide advice if you have experience in doing this. Thanks.

You don't mention which version of Ubuntu you're using. If you're using 18.04, there are a couple of things you can try.

The first thing to do is to make sure the resolution of 1920 x 1080 is supported in grub. To do that reboot your computer and hit Esc repeatedly until you get the purple screen with the recovery and grub options. Hit "c" to get a grub prompt. At the grub prompt, type:

videoinfo

and hit Enter.

Look at the list of "supported" resolutions, and make sure 1920 x 1080 is there. If so, type:

exit

to exit grub and boot.

The boot screen resolution is changed by changing your default grub settings. Open a terminal and enter:

sudo nano /etc/default/grub

Use the down arrow or Page Down until you see the line that looks like this:

#GRUB_GFXMODE=640x480

Below that line, enter the following, substituting the 1920x1080 for a supported resolution:

GRUB_GFXMODE=1920x1080
GRUB_GFXPAYLOAD_LINUX=keep

To save your changes, hit Ctrl+o, with "o" as in Ohio, not zero. To exit nano, hit Ctrl+x. Now, update grub:

sudo update-grub

That should solve the problem in grub, and won't be overwritten by an update. For the login screen, you may be able to fix it with the 4K settings in Ubuntu 18.04. Go Settings -> Devices -> Displays and set "Scale" to 200%. I'm not sure if that will affect the desktop manager or not, but it's the first thing I'd try.

Just want to add that I found a way to change the login screen resolution. That part of my problem has been asked and answered, see how to change gdm3 thread.

After setting up the monitor resolution and zoom level I wanted, I simply copy the settings to gdm3 .config directory, make any further changes you need and then reboot the PC.

sudo cp -i .config/monitors.xml /var/lib/gdm3/.config/
sudo vi /var/lib/gdm3/.config/monitors.xml

You probably also need to do the following before rebooting. Select gdm3 when prompted.

sudo dpkg-reconfigure gdm3