[ruby-on-rails-3] How can I change cols of textarea in twitter-bootstrap?

The other answers didn't work for me. This did:

    <div class="span6">
      <h2>Document</h2>
        </p>
        <textarea class="field span12" id="textarea" rows="6" placeholder="Enter a short synopsis"></textarea>
        <button class="btn">Upload</button>
    </div>

Note the span12 in a div with span6.