Mplayer Full Screen Tip
However, there's a very simple fix: simply edit your mplayer configuration file. In Ubuntu, it looks like this:
username@computer:~$ cd .mplayer
username@computer:~/.mplayer$ sudo gedit mplayer.conf
To the end of the file, add the line:
All done. Once you start mplayer the next time, it'll go to full screen mode.