My xrandr does not support the "Border" prop for HDMI-0.
Attempting this will result in the following error:
$ xrandr --output HDMI-0 --set Border 100,0,100,0
X Error of failed request: BadName (named color or font does not exist)
Major opcode of failed request: 140 (RANDR)
Minor opcode of failed request: 11 (RRQueryOutputProperty)
Serial number of failed request: 41
Current serial number in output stream: 41
However to "shrink" the screen to give yourself black bars, sometimes referred to as pillarboxing, one method is to set a resolution which does not take up the full width of the monitor (e.g. 1024x768 w/ aspect ration 4:3) then set the 'scaling mode' with xrandr
xrandr --output HDMI-0 --set 'scaling mode' 'Full aspect'
This gives me black bars on sides when screen is too wide. I think this will work with any resolution with aspect ratio 4:3. You can also check here for using xrandr to add resolutions not available by default in the display setting.
xrandr --prop for other properties that might help.
EDIT: for devices which do not support 'scaling mode' 'Full scale', you can use an xrandr transform.
xrandr --output HDMI-1 --mode 1024x768 --scale 1.33333333x1 --transform 1.33333333,0,-100,0,1,0,0,0,1
to create pillar box style black bars on the sides. Adjust -100 to be the width you want for the bars.