You could try styling it with CSS by adding an ID or a class to the textarea and inserting styles into the wp_admin head.
Or, a quick way would be to do something like this :
echo '<textarea name="_dresscode" class="widefat" style="width:400px !important; height:80px !important;" >' . $dresscode . '</textarea>';
If not, have you tried simply :
echo '<textarea name="_dresscode" class="widefat" cols="50" rows="5">' . $dresscode . '</textarea>'