<!DOCTYPE html>
<html>
<head>
<script>
function changeSize() {
document.getElementById("mySelect").size = 4;
}
</script>
</head>
<body>
<form>
<select id="mySelect">
<option>Apple</option>
<option>Banana</option>
<option>Orange</option>
<option>Melon</option>
</select>
<input type="button" onclick="changeSize()" value="Change size">
</form>
</body>
</html>