Command Line – How to Reset Display Settings Through Terminal

The problem seems to be that you are not able to see/control the lower part of the "Display settings". There is a workaround to move the lower part on top. For this, go to System settings -> Display and then right-click on the window bar on top and click on "Move". Do not move your mouse after clicking on "Move", otherwise the keyboard is not used for moving, only the mouse has the control for moving the window.

and then use your keyboard to move the window to top, slowing showing you the lower part where you drag your mouse to reset the scale to 1. Remember to click "Apply" for the settings to persist and then click on "Keep this configuration".

You can also hold down the Alt key and then click on any area of the window to move it up until you can see the slider.

In case anyone else runs into this, this is what I did (though I imagine there must be a better way): I got a spare monitor and connected it to the computer (after turning it off), along with the original monitor. (Fortunately my video card had multiple plugs.) On rebooting, I used nvidia-settings to move the new monitor's position to below the other one. That gave enough room for all (or at least enough) of the System Settings window to be visible, so I could get to the Scale Display bit and reset it back to 1. Then I shutdown the computer and removed the extra monitor. On restart, the GUI was back to normal size and I could see all of the System Settings window.