You can do it with xrandr
to set your monitor to 50% scale. This is not permanently set so you would have to set it into a startup script if you want to keep it that way on every bootup.
First, get your currently connected monitors with the following command:
xrandr --current | grep -w connected
Example:
xrandr --current | grep -w connected
DVI-I-0 connected 1280x1024+1920+37 (normal left inverted right x axis y axis) 0mm x 0mm
HDMI-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 509mm x 286mm
Pay attention to the monitor that you want to scale. I am going to scale my second monitor that is attached to my DVI port. 50% is .5; 100% is 1 So we will run the following command to scale to 50%.
xrandr --output DVI-I-0 --scale .5x.5
If you want it to change on every startup, add that command to your Settings -> Session and Startup but you might want the command like so to give it a slight delay at startup:
bash -c 'sleep 5; xrandr --output DVI-I-0 --scale .5x.5'
To set the monitor back at any time run the following:
xrandr --output DVI-I-0 --scale 1x1
Hope this helps!