If you have a widescreen movie and you do not want to scale it to full height, you can use the expand=w:h filter to add black bands. To view a 640×384 DivX, try:
mplayer -ao mpegpes -vo mpegpes -vop lavc,expand=640:576 file.avi
If your CPU is too slow for a full size 720×576 DivX, try downscaling:
mplayer -ao mpegpes -vo mpegpes -vop lavc,scale=352:576 file.avi
If speed does not improve, try vertical downscaling, too:
mplayer -ao mpegpes -vo mpegpes -vop lavc,scale=352:288 file.avi
from http://web.njit.edu/all_topics/Prog_Lang_Docs/html/mplayer/video.html