<select>
<option value="value1">選項1</option>
<option value="value2">選項2</option>
<option value="value3">選項3</option>
</select>

select:hover {
height: 100%;
}

select {
position: relative;
}
select option {
position: absolute;
top: 100%;
left: 0;
}

select option {
...
background-color: #fff;
border: 1px solid #ccc;
box-shadow: 2px 2px 5px rgba(0,0,0,0.3);
...
}