If your computer is too slow to process the image and the sound of a large mkv file,
you can resize by re-encoding to a lower rate :
mencoder /path/orginal.mkv -oac copy -ovc lavc -lavcopts vcodec=mpeg4:vhq:v4mv:vqmin=2:vbitrate=2000 -vf pp=de,scale=1280:-2 -o /path/result_file.avi
I choosed a 1280 width because of my screen and a better vbitrate than a normal divx (~700Mo)
so the quality is good !