mplayer -vop crop=w:h:x:y <filename>

also try

mplayer -vop expand=w:h:x:y <filename>
(expand stretches the video rather than cropping it)

Where "w" is the width of the screen, "h" is the height, and "x" and "y" are the off-set from the upper left corner of the image

There should be a way to do this automatically... let me check

Check the MPlayer manual page for more info: http://www.mplayerhq.hu/DOCS/man/en/mplayer.1.html