[eluser]Pascal Kriete[/eluser]
[Edit: Ignore this, beat by the robot - need to learn to refresh]
You can, just pass a string of additional parameters to the function as it's 4th parameter.
Code:
echo form_dropdown('shirts', $options, 'large', 'id = "select1"');
What you can't have is an individual id on each option, you would need to extend the helper to achieve that.