To change the font size in XMBCBuntu (formerly known as XBMC Live) you have to edit the /etc/X11/xorg.conf file.
Login to the machine via ssh (and enter your password):
Once you logged in enter the following command (you will be prompted for your password):
sudo nano /etc/X11/xorg.conf
Afterwards a kind of text editor will be opened and add the following line to the “Monitor” section.
Option "DPI" "96 x 96"
Close the editor via “CTRL + X” and save the file