It’s possible to instruct browsers to render video elements in different sizes for Dawn (or other themes) by using media queries to set up rules for specific screen sizes. With media queries you can use the max-width CSS property to target devices with screens smaller than a certain width. Since your videos are being included in the custom liquid section, you can add media queries to the existing CSS, so it would look something like:
The properties for tablet/ mobile max-widths are just placeholders and you can change them to work as you’d like. You’ll also need to test the video display on different devices or use browser developer tools to simulate different screen sizes.