How can I set wp_dropdown_users so that it shows only authors?

Question

you can create a function which gives you all users which have role author in a drop down list.

Note: It is a dummy example modify it as you want( put this function in functions.php or custom plugin ).

Use: Call this function any where in theme like <?php ravs_author_dropdown_list(); ?>

function ravs_author_dropdown_list() {
    // query array
    $args = array(
        'role' => 'author'
    );
    $users = get_users($args);
    if( empty($users) )
      return;
    echo'<select>';
    foreach( $users as $user ){
        echo '<option value="'.$user->data->user_login.'">'.$user->data->display_name.'</option>';
    }
    echo'</select>';
}