Did a quick test on the new BG image feature and it seems to only display at 500px height.
Added a section, then dragged the BG video element into it. Section was set to 100% height, 100VH, 800px, a whole bunch of stuff, yet the video always displayed at 500px.
Is this by design? There appears to be no setting for this feature other than the upload button.
Can you please update your post with some more information so we can help you faster? Things like screenshots and your environment info really speed things up.
You are right, adding a class and adjusting the video works. I had set my test header to be 500px assuming the settings for BG video would be in the feature settings similar to the way sliders are handled, or placed image settings are controlled within the settings.