Depends on what format the movie is in now. If it is dvd-video, you can use a program like DVDShrink or Nero Recode to reduce the size of the video to fit the disk. With a lot of compression, the video quality may suffer though, especially using a transcoder like Shrink. On large dvds, you can split them onto two disks, or reencode with a good encoding program. Splitting saves the quality but is inconvenient on playback. Reencoding quality depends in part on the program you use. I like DVDRebuilder to reencode dvds…and there is a free version that uses a very good encoder called HC.
If the video is some other format, say an avi, you can reencode using a smaller bitrate (and or resolution) to reduce the size of the video. AutoGK is one program that can be used with xvid or divx, MeGui is one that can be used with xvid or H264. You would need a dvd player that is capable of playing divx or xvid…the H264 has greater ability to reduce the size, but cannot be played on a dvd player. It is possible to encode to make it work on a Blu Ray player, but that is fairly involved…look up the process at videohelp.com or doom9.org.