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

I don't know if this is the correct way however I did this:

<div class="control-group">
  <label class="control-label" for="id1">Label:</label>
  <div class="controls">
    <textarea id="id1" class="textareawidth" rows="10" name="anyname">value</textarea>
  </div>
</div>

and put this in my bootstrapcustom.css file:

@media (min-width: 768px) {
    .textareawidth {
        width:500px;
    }
}
@media (max-width: 767px) {
    .textareawidth {

    }
}

This way it resizes based on the viewport. Seems to line everything up nicely on a big browser and on a small mobile device.