π» XrandR
cvt 1600 900
Whatever resolution you need goes here 1920x1080, 3840x2160, etc.
Copy line that starts with Modeline
sudo xrandr --newmode [...copied line minus the word Modeline...]
The –newmode flag or optarg notifies the system of a new Modeline
sudo xrandr --addmode {vga,Virtual}-{0,1} "1600x900_60.00" #Virtual-1
Included Virtual for Virtual Machines (Slackware can be tricky)
sudo xrandr --output Virtual-1 --mode "1600x900_60.00"
Whatever output display and mode needed
ARandR or System Settings > Display > select new option generated: 1600x900
If you don’t see the newly generated option reboot and check again