In Ubuntu 11.10 (or before?), in “appearance” there’s no longer an option to change the font size (why does Ubuntu introduce every now and then a new lack of customization?). In order to do it now we have to install Gnome Tweak Tools:
sudo apt-get install gnome-tweak-tool
you can also look it up in the Ubuntu Software Center as “Advanced Configuration”. We open it and in the “Typography” section we can alter the global fonts used by the system.