Do you mean screen resolution? If so, it depends on the video source - if you're watching a regular DVD, for instance, that's standard def, usually 720x480 (NTSC). HD movies like Blu-ray, on the other hand, are often 1080p, which is 1920x1080.
If you're thinking about reducing your desktop resolution to watch a movie because the movie is of a lower resolution than your current desktop settings, don't bother. It's usually a better idea to just scale the movie up to fullscreen in your media player.