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 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