How to change resolution of login screen on system boot

To change mode the greeter uses, you can put this, substituting any mode that xrandr shows your display supports (3840x2160, 1920x1080, 1600x900, whatever), in xorg.conf:

Section "Monitor"
...
	Option	"PreferredMode"	"2560x1440"
...
EndSection

Alternatively, PreferredMode can be put in 10-monitor.conf.