How to apply css styles to codeigniter dropdown?

form_dropdown('dropdown', $options, $selected, 'style="width: 240px; font-size: 13px"');

You could just as easily add a class to the  and then add the styles in your stylesheet:

form_dropdown('dropdown', $options, $selected, 'class="foo"');

In either case, if you don't have a value for $selected, set it to array()

form_dropdown('dropdown', $options, array(), 'style="width: 240px; font-size: 13px"');

Although you could get away setting it to null or '' for convenience.