We measure the height and width of an element in pixels, like 50px. Pixels are tiny points that make up what you see on a screen.
<head>
<link rel="stylesheet" href="style.css">
</head>
<body>
<h3>Two different theories</h3>
<img class="round" src="https://mimo.app/r/earth.png">
<img class="flat" src="https://mimo.app/r/earth.png">
</body>
.flat {
height: 50px;
width: 100px;
}
.round {
height: 100px;
width: 100px;
}