Sublime Text 2 prevents you from decreasing the font size below the point of readability. However, it may still be useful to make the font smaller if you don't need to read individual words or characters, such as when you are using Sublime to format or organize data.

To decrease the font size, open ~/.config/sublime-text-2/Packages/Default/font.py

Find the lines:

if current < 8:
  current = 8

And modify them accordingly.