Video on phones

Hi,
I would like to ask for help: I have my website and when I try to access it on my phone, it shows video exactly in the same size like on laptop so I can only see 1/3 of the full video on the small phone screen. What can I do to adjust automatically size of the video when someone is accessing the website on mobile phones? I just want to say that I don’t want to change the theme so I am looking for any codes etc