issue: fullscreen mode does not work, the screen just gets filled with black around the video image which stays small
fix: make sure you are using "xv" as the video output driver ("-vo xv" on the command line)
issue: aspect ratio is not being maintained in fullscreen mode
fix: figure out what your screen's aspect ratio is (resolution width / resolution height) and then use this value for "monitoraspect" in the mplayer config file. default monitoraspect is 4:3. if you resolution is 1280x1024, use monitoraspect=5:4.