This applies to ebedding both youtube and facebook videos in Video Share – Joomla Extension. You must create a thumbnail image for the video or it will not post the video.
Embedding videos allows for the default player to load the video. If you use Video Share youtube inclusion the Video Share player shows the video but, when a user clicks the video then the default video player opens and you can not size the video.
So you can size the video in the Video Share Player Settings for embeded videos.