<label for="cars">選擇車型：</label>
<select id="cars" name="cars">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="mercedes">Mercedes</option>
<option value="audi">Audi</option>
</select>

<style>
select {
width: 200px;
}
</style>

<style>
select {
width: 100%;
}
</style>