The video should be 20 seconds minimum and 90 seconds maximum and preferably loop-able. 40 seconds seems to be the sweet spot. As for the size this is for the web smaller is better a general rule is 720p and with a bitrate under 700 kb/s ( preferably 500kb/s ).
With Google’s recent changes to Youtube embedded video player we only recommend using the external video cover option. This requires the video be in mp4 format and optimized for web.
Also, can can provide a static frame/image for those that might not get the video. This will include all mobile device users as video is not supported on those devices.
Changes to cover videos using YouTube’s video player effective 11/2018
Google recently announced a couple of changes to their YouTube player API. One of the changes is to the showinfo API parameter. Before this change it was possible to remove the channel avatar and video title before the video started playing. This parameter has been deprecated. Following the change this information will always be displayed before the video, when paused and after the video ends.
For this reason we will be phasing out the YouTube Cover Video Block as currently customers using the block are switched over.