If you experience different font sizes whenever you plug a different monitor to your laptop, maybe you are hit by Xorg's Xserver default behavior to detect the dpi settings from the current DDI data of the screen. This gives in most cases not the expected result for the user, thus forcing the Xserver to a fixed dpi setting is a good idea.

Under KDE you can force the font engine to use fixed dpi setting (96, or 120). But if you are using non-KDE applications too, you may want to set it to the running Xserver directly. Unfortunatelly, there is no option in xorg.conf to do that, so you have to modify you session start script and add the -dpi option of the X server executable.

For KDM 4.x edit the config file (under Gentoo: /usr/share/config/kdm/kdmrc) and change the line:

ServerArgsLocal=-nolisten tcp

to:

ServerArgsLocal=-nolisten tcp -dpi 96