To set the font size, use the fontSize property in JavaScript. You can try to run the following code to set the font size of the text with JavaScript −
Example
<!DOCTYPE html>
<html>
<body>
<h1>Heading 1</h1>
<p id = "myID">
This is Demo Text. This is Demo Text. This is Demo Text. This is Demo Text.
This is Demo Text. This is Demo Text. This is Demo Text. This is Demo Text.
</p>
<button type = "button" onclick = "display()">Set Font Family and Size</button>
<script>
function display() {
document.getElementById("myID").style.fontFamily = "verdana,sans-serif";
document.getElementById("myID").style.fontSize = "smaller";
}
</script>
</body>
</html>