How do I customize Gnome screen shield / curtain / login screen appearance?

Not sure if this is a little too late but you can customize the values in /usr/share/gnome-shell/theme/gdm3.css and that does the trick. Just find the screen-shield-clock class and make your changes. One simple thing I did to make it a little nicer is set the font-weight to 300 and it improves the look a million times.

Ubuntu

I use ubuntu 18.04, (also done this in 17.10) and as of how @cezanee-vahid said, you can find the CSS file here: /usr/share/gnome-shell/theme/gdm3.css and by changing this file you don't need to restart gnome or do anything else, they will be changed by locking computer (or better to say will be reloaded).

so for changing clock size you only need to change font-size and font-weight for .screen-shield-clock-time class.

also for if you want to get ride off that arrows in the bottom of the screen just set 0 for height and width of .screen-shield-arrows Gjs_Arrow CSS selector. setting 0 for font-size of .screen-shield-clock-date will do the same for Date.

Arch

I'm not an Arch or even Arch-based distro user, but after reading ArchWiki about GDM it seems like you need to compile source files for GDM after changing them. also, the gdm3.css file in Arch is named gnome-shell.css

so in conclusion, these spets are needed to change GDM theme in Arch:

1. extracting the current GDM theme
2. creating a gnome-shell-theme.gresource.xml file for the selected image files
3. changing gnome-shell.css file as you wish!
4. compiling the new GDM theme you have created
5. replacing files under /usr/share/gnome-shell
6. restating gdm.service

the ArhcWiki above have explained these steps detailed.