"Why is mplayer display so small and can it be adjusted"
 

Read somewhere that I needed to set the video driver to "XV" but I'm so new to Linux that I don't know how to do this or if it will even help.

I believe you fire up mplayer and right click on the controls. I think you can set preferences there.

Try to launch mplayer with this command:
Then you should be able to switch to full screen mode etc. Good luck!

--Simen

In command line:

gmplayer -zoom
Use the 'f' key to toggle between full screen and the small size.