I use an X1 Carbon for a few months now.
I install Ubuntu 18.04 on a Micro Usb Drive.
With the screen size and resolution, I need to scale the UI.
But by default Ubuntu option are 100% and 200% witch is too small or way too big.
If you need to scale the UI to let say to 125%, use this command in terminal.
gsettings set org.gnome.desktop.interface text-scaling-factor 1.25
The last number is the scale. 1.25 = 125%