A while back I was asked to help a professor who was trying to play video in a presentation for a conference. While they were struggling to get the file to play – I fixed that in just a few minutes – I was worried that they were using a very small video on a larger slide. It just wasn't going to look good.
For the best effect, videos should be played fullscreen. This can be done by simply scaling the video until it fills the entire slide, as you do with images.
Just like images, videos should have a resolution of at least 800x600. This isn't always possible, since that resolution is set when the video is created. In this case, the videos were fairly small, but I came up with a trick to give the illusion of playing the video fullscreen.
I simply set the slide background to black and scaled the video as large as it would go without becoming to pixelated.
The perfect solution would be to reformat the video from the original source to match the screen resolution for the presentation. This usually isn't possible since most teachers don't shoot their own video. However, redesigning the slide is another great alternative.