This site is mobile accessible. Press the "Tap Here" button to use a smaller font-size.

Smartphone icons created by Freepik - Flaticon

Change the font size of an element on different screen sizes

Example DIV.

When the browser's width is 600px wide or less, set the font-size of DIV to 30px. When it is 601px or wider, set the font-size to 80px. Resize the browser window to see the effect.