You probably mean the Distraction Free Writing mode. Am I right? Then you changed wrong ID, it’s #wp-content-editor-container what you want to edit. But every time you hack WordPress core, a kitten dies. So if you like kittens, here is no-offensive solution for you. Put this code into functions.php file in your theme:
<?php
add_filter( 'admin_footer', 'fulleditorwidth', 99 );
function fulleditorwidth() {
?>
<style type="text/css">
#wp-content-editor-container { width: 1024px!important }
</style>
<?php } ?>
I recommend to add also max-width: 100% for responsive view.
Don’t hack your WordPress core files. If your wordpress will be updated, your changes will be overwritten.