When looking for video to include in a multimedia project, it is important to be aware of how resolution and compression can impact the aesthetic of your piece. Resolution is the number of pixels running horizontally and vertically in your image.
The standard resolution for a project that combines video, image and audio elements is HD (1920x1080). If you don't want your video to have artifacts, make sure the files you download are at least 1920 pixels x 1080 pixels. You can check your files resolution by opening in a media player or video application and checking the properties of the video.
Some websites that allow download of videos also give the option for different formats. Since these files are usually heavily compressed, downloading the larger file type typically (not always) means it will be less compressed.