The WYSIWYG uses the .wp-editor-container class. So the easiest way would be to change this in your css. If your style is being over written then just add !important.

    .wp-editor-container {
    width:50%; // What ever size you want
    }

the wp_editor function also allows us to pass a class to the <textarea> not sure if this might help you. Here's how tho:


    <?php  <?php wp_editor( $content, $editor_id, $settings = array('editor_class'=>'yourclass') ); ?>

You can take a look at the documentation here: [wp_editor - Codex][1]


  [1]: http://codex.wordpress.org/Function_Reference/wp_editor