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

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