How can I make my video square (increase height) for mobile only?

[Nawshin1992]

Hi everyone! When I upload a video on to my Shopify store it always looks great on desktop. However, the same format on mobile makes it look small. I want to know how I can increase the height of the video (and decrease the width) so that it takes on a more mobile-friendly square shape. I was told I can do this with a bit of CSS. Can anyone kindly give me instructions? And how can I ensure these changes are ONLY for mobile and not desktop? Please keep in mind I am a complete Shopify-noob and you'd have to break it down to me like I'm a dummy! Thank you in advance!

[Nawshin1992]

Another thing to keep in mind, my Shopify store theme is called "Ella".