Resizing mobile background videos across multiple pages

Site URL: https://buffalo-flounder-dxyp.squarespace.com/

Hi SS community,

I want to -

Resize background videos for mobile, so they reduce in height to a standard 16:9

The videos are embedded via Vimeo links.

I don't want the video to crop in height, I want the whole video to scale down to fit mobile.

I also want to do this via a single piece of code that I can apply to all project pages, as every project will have a video playing at the top of the page. 

Is this possible? Every piece of custom CSS I have tried has failed...please help!