This is one way you can accomplish this:
In your youtube embed code add this to the iframe tag:
style=“position: absolute” width=“100%” height=“100%”
Now the Youtube video will automatically scale to the size of the parent container. In your case it will be the “HTML Embed Element” that Webflow has, which you can style as you normally would.
In order for this to work you need to make sure the parent element (the HTML Embed Element) has the position set to something. It cannot be set to auto or the iframe will ignore it.
Here’s a quick example I put together: https://preview.webflow.com/preview/youtube-example?preview=c81346ec042dd94d72762ce58dc7f2b8
I can’t know for sure if this will work for you since I can’t see your project, but hopefully this helps 