JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-2.2.4.min.js"></script>
<p>
  <select id="firstSelect" multiple="multiple">
    <option>one</option>
  </select>
</p>
<p>
  <select multiple="multiple" size="return this.length();">
    <option>one</option>
    <option>two</option>
  </select>
</p>
<p>
  <select multiple="multiple" size="return this.length();">
    <option>one</option>
    <option>two</option>
    <option>three</option>
  </select>
</p>

JavaScript

$('#firstSelect').attr('size', $('#firstSelect').length);