Down Size YouTube Video

Hi I just startet with this Theme and now all the videos are Full Screen even with Width = 600 and Height 400 On the visual in the back end it seems okay. The size. However at the Front End the Video is Full Screen or a bit more. Impossible for users to watch. Example: https://www.masterchessopen.com/nimzowitsch-larsen-attack-a01.html How can the YouTube Video be sized to Width = 600 and Height 400 ? All the best Claus