这是不可能的.您只能实现自己的选择框，但这不利于可用性.

This is not possible. You can only implement your own select-box, but this is bad for usability.

另一种方法是以编程方式更改选择框的大小属性，但这并不是您真正想要的.

another approach is to programmatically change the size-attribute of the select-box, but this is not really what you wanted.

我建议考虑一下您为什么需要这个，以及是否有更好的软件模式?

I suggest to think of why you need this and if there would be a nicer software-pattern?