JSFiddle - React, Tailwind, and code Playground

HTML

<div id="myDiv" class="medium"></div>
<button id="btn">Click me</button>

CSS

#myDiv {
    background-color: gray;
}

.medium {
       height: 50px;
       width: 50px;
}

.big {
       height: 100px;
       width: 100px;
}

JavaScript

document.getElementById("btn").onclick = function() {
    var element = document.getElementById("myDiv");
    
    if (element.className == "medium") {
        document.getElementById("myDiv").className = "big";
    } else {
        document.getElementById("myDiv").className = "medium";
    }
};