That's an interesting question. The answer depends on the video source that you're compressing. If the source is a dual layer DVD (or similar large good quality source) which uses a high-bitrate for the entire movie, then the chances are that you can compress it a fair bit without noticeable loss of quality using a variable bitrate mode where the bitrate remains high for fast action sequences and scene changes, but is lowered when the video is (almost) still. If, however, the source is already highly compressed in this manner, then reducing the bitrate further will reduce the picture quality noticeably.
The other thing that you can do to reduce the size of a given movie is to re-encode it with a more efficient codec such as H264 which can produce better results at lower bitrates, but this is only useful if you use a PC for playback or have standalone equipment that supports H264 containers such as mkv and mp4.