I have a similar question with 16.04 with nvidia based graphic card to a Panasonic 26" TV,detected as an 32" if connected HDMI.
I could solve the Output Problem, i can see the hole screen with the "underscan" Option, but this doesn't solve the reason for this Fault.
The output of "xrandr" is stillgiving the worngphysicalMonitor size and i can't find the correct "xrandr" semantic to change the Monitor size to its correct values.
This is the "xrandr --query" output:
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
DVI-I-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 697mm x 392mm
1280x720 50.00 + 60.00 59.94
1920x1080 60.00 50.00 50.00 59.94*
1920x1080i 60.00 50.00 59.94
720x576 50.00
720x480 60.00 59.94
640x480 60.00 59.94
VGA-1 disconnected (normal left inverted right x axis y axis)
The values "697mm x 392mm" should be changed to 575mm x 325mm.
I guess this would do the job the "right" way.