JSFiddle - React, Tailwind, and code Playground

HTML

<div id="theBox"></div>
<span id="lblBoxSize"></span>

CSS

div {
    background-color: gray;
    width: 32px;
    height: 32px;
}

JavaScript

var theBox = document.getElementById("theBox");
var lblBoxSize = document.getElementById("lblBoxSize");

lblBoxSize.innerText = "("+theBox.clientWidth+","+theBox.clientHeight+")";