My new Dell XPS 13 has a HiDPI touchscreen and when I plug a 2nd display (not HiDPI), the resolution is big and ugly.

Solution for this is using the –scale option of xrandr!

My script example (in this case, laptop screen is at the right of my external screen):

xrandr --output eDP1 --primary --mode 3200x1800 --pos 3840x192 --rotate normal --output DP1-1 --mode 1920x1080 --pos 0x0 --rotate normal --scale 2x2