My laptop's graphics card is a dedicated 4GB AMD Radeon R7 M270. The native resolution is 3840x2160 (16:9). On a 15-inch monitor, everything on the Ubuntu login is tiny. I needed to adjust the settings to a supported legible resolution.
# Mind the connected display from the console output
$ xrandr -q
$ sudo gedit /etc/lightdm/lightdm.conf
# Set these configuration options:
> [SeatDefaults]
> display-setup-script=xrandr --output eDP1 --primary --mode 1920x1080
# Where:
# eDP1 is the connected display
# 1920x1080 is the working resolution
sudo reboot