Typically this is a matter of how the video was encoded in the first place, make sure your video has a wide screen aspect ratio on your encoder. First though, make sure your device is either in HDTV mode or in SDTV 16:9 mode, and double check that your TV is set to the proper mode for wide screen video. Different TVs handle it differently, for example, my Vizio adapts correctly to whatever mode my Roku is in without my having to change anything, but others (including the same brand) require manual changes to work in the correct mode.
- Joel