When there is no configuration option for changing the default font type and size in Ubuntu 12.04, try the below steps:
1. Instal gnome tweak tool:
sudo apt-get install gnome-tweak-tool
2. Start gnome tweak tool by typing command "gnome-tweak-tool" in the terminal.
3. GNOME Tweak Tool window will appear
4. Select the Fonts section on the left
5. Choose your preferent default font, document, monospace and window title font as well as font size and hinting and antialiasing types.
