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 overwritten then just add `!important`.

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

the `wp_editor()` function also allows us to add a class to the `textarea` tag, I'm not sure if this might help you. Here's how though:


    <?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