Any video in the timeline will be scaled to the size of the rendered movie when you render your final slide show. For best results, your video should be at least as high resolution as your output movie format.

For example, if you're producing a movie for the web which will be 640x360, then any video you use in the timeline should be at least that large for best results.

In addition, you can choose how the video is scaled to its enclosing frame if it is a different aspect ratio. The video can either be zoomed to fit in which case some of the video may be cropped; or it can be scaled down to fit within the enclosing frame in which case there may be background showing around the top/bottom or left/right edges of the video.