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How do I change the size of the videos in the lightbox?

You need to add the desired width and height to the video URL. For example for a YouTube video with the URL http://youtu.be/oeuHxC1cF0o you would add the following to the end of the URL &width=500&height=250 to give http://youtu.be/oeuHxC1cF0o&width=500&height=250. This will give you a video with a width of 500px and a height of 250px.