To make your existing countdown, you need to add codes. This blog post shows you an example of taking an existing countdown and adding codes to make it: https://w3schoolweb.com/how-to-make-a-countdown-timer-in-html-code/
You can change at what size you want your countdown to change. But the blog post should give you a great start.
Here’s another similar article: https://w3schoolweb.com/date-time-countdown-timer/ https://w3schoolweb.com/countdown-html-css-js/
This articles will help you : https://w3schoolweb.com/