<!DOCTYPE html>
<html>
<head>
<style>
option:checked {
height: 100px;
width: 100px;
}
</style>
</head>
<body>
<select>
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="opel">Opel</option>
<option value="audi">Audi</option>
</select>
</body>
</html>