Change dropdown width

Which dropdown do you mean?

Ah, i understand. you are doing a backend interface for your plugin?

http://wpengineer.com/wp-content/uploads/screenshot-2.png
Here you find a list of all needed elemets including their style classes. For your exact question: You can give your select your own class and define the width by CSS.

For Example: <select class="my-select-class">

Then you have to define your own stylesheet CSS file for this.
Here is the way you can attach the stylesheet to the admin area:

function admin_register_my_styles() {
    $siteurl = get_option('siteurl');
    $url = $siteurl . '/wp-content/plugins/' . basename(dirname(__FILE__)) . '/yourstyle.css';
    wp_register_style( 'your-plugin-slug', $url );
    wp_enqueue_style( 'your plugin-slug' );
}
add_action('admin_print_styles-[YOUR OPTIONS PAGE SLUG]', 'admin_register_my_styles');

I hope that helped?

No Problem 🙂

The form fields from contact form 7 work the same way. you can give them classes like you want. For Example: [select choosesome class:my-class]

Then you can define the width via CSS in your themes style.css file.