The file size entirely depends on the bitrate (Quality/Resolution) and length of the video. I genrally use Divx or MPEG4 for backing up DVD movies on my computer. These codecs offer great compression and speed and I think a video of same resolution and length will result in a smaller file size when encoded with divx compared to wmv. I have somtimes seen that mpeg4 or mp4 files give a better compression than divx and have been using that for about a year now.