How do I change the font DPI settings?

Gnome Tweak Tool

This is a common issue on the forums - the move to gnome3 has dumped many of the customisation features that were available in gnome2.

One GUI tool that exists in Software Center is gnome-tweak-tool - its a partial solution to allow you to customise some aspects of fonts.

The "Text scaling factor" is the option you are interested in - its unfortunate that it is a sliding bar so you will not be able to enter the actual DPI value. Click on the sliding bar and use the left/right arrow keys to decrease/increase the font size.

dconf-editor

Using dconf-editor which is available in the dconf-tools package allows you to set the "Text Scaling Factor" numerically i.e. changing the default value by fractions of numbers (1.2, 0.9 etc) changes the overall screen font size:

Universal Access

If you just want to adjust the text size universally without need detail you can do this from the universal access tool:

Just open gconf-editor , navigate to the key /desktop/gnome/font_rendering/dpi and adjust to values you need.

If the key text-scaling-factor is missing from gconf-editor, check alternatively:

 gsettings list-schemas | grep org.gnome.desktop.interface
 gsettings list-keys org.gnome.desktop.interface
 gsettings get org.gnome.desktop.interface text-scaling-factor
 gsettings set org.gnome.desktop.interface text-scaling-factor 0.9

(replace 0.9 with the desired value).

Did you try setting DisplaySize in xorg.conf?

I added to /usr/share/X11/xorg.conf.d a file with the size in mm, let's see how it works:

Section "Monitor"
  Identifier "myMonitor"
  DisplaySize 223 125
EndSection