Unity, a default desktop shell in newer Ubuntu releases doesn't have an option for changing default font type and size as Ubuntu had in the previous versions.
Somehow, I think that default font size in Ubuntu 12.04 Precise Pangolin is too big for modern screen resolutions. Here are instructions how to change default font size in Ubuntu:
1) Instal gnome tweak tool with the command:
sudo apt-get install gnome-tweak-tool
2) Start gnome tweak tool either by click on Dash Home and entering "advanced settings" or typing command "gnome-tweak-tool" in the terminal
3) GNOME Tweak Tool window will appear4) Select the Fonts section on the left5) Choose your preferent default font, document, monospace and window title font as well as font size and hinting and antialiasing types.