代码示例：（标识：bs_form_select_size）
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link href="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/5.1.2/css/bootstrap.min.css" rel="stylesheet">
<script src="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/5.1.2/js/bootstrap.bundle.js"></script>
</head>
<body>
<div class="p-4">
<h2>选择菜单的大小</h2>
<p>请使用 .form-select-lg 或 .form-select-sm 类来改变选择菜单的大小：</p>
<form>
<select class="form-select form-select-lg">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
</select>
<select class="form-select mt-3">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
</select>
<select class="form-select form-select-sm mt-3">
<option>1</option>
<option>2</option>
<option>3</option>
<option>4</option>
</select>
</form>
</div>
</body>
</html>
