Here's how I just fixed that on an Ubuntu Linux machine:
As root, edit /etc/X11/xorg.conf. In the "Section "Device"" part of the file that has your video card settings, put this so it won't try to get the DPI from the monitor:
Option "UseEdidDpi" "off"
Then, in the "Section "Monitor"", I added this line:
DisplaySize 508 286
That makes my 1920x1080 display use 96x96 dpi, a good common value.
This thread in the Arch linux forums lists this formula for the DisplaySize: displaysize = (
Save the file, restart X with control-alt-backspace, and see if it works.