If you wish to change the font of the text in your Matlab figure globally, then you have to set :
>>set(gca, 'FontSize', xx);
And if you want to change it only for a specific legend or text or axis-handles just add the option 'FontSize', xx to your parameters.
But this did not work in my Ubuntu 12.04 system. No matter what changes I made, Matlab displayed a fixed font size.
Thanks to the following forums, I could sort it out:
You need to go to terminal and install following two packages, log in again.
$sudo apt-get install xfonts-75dpi
$sudo apt-get install xfonts-100dpi