I notice that there seems to be a strange effect happening in Mozilla depending on how this setting is set, how X is aware of the exact size and resolution and pixel dimensions of the display, and what settings have been modified in the .js Mozilla settings file 'unix.js'.
I've managed to get a different Linux install (not Knoppix) to display absolutely enormous Mozilla fonts everywhere - not what I want - except within the page viewport of the Mozilla web browser, where font sizing seems correct - which is how I want.
I think something somewhere is getting confused as to what constitutes a 12 point font and whether this might have anything to do with a 12 pixel high font, in a system where the TFT monitor gives me a resolution of 85dpi, which is what my XFree86 system is set at - as far as I'm aware (I'm still learning a lot about this particular area).
Obviously, the relationship between point size departs from pixel height depending upon the font, the aspect, the display resolution (dpi), whether the display resolution is an accurate derivation of the actual measurable size of the display, and the user-agent's idea of accuracy. And probably the wind speed.