Quickly add CSS for block editor

functions.php.php

<?php // Don't include this opening tag

/**
 * Add inline styles to admin head
 */
add_action( 'admin_head', 'child_block_editor_styles' );

function child_block_editor_styles() {
	if ( 'post' == get_post_type() ) :
	?>
<style>
	.block-editor-writing-flow .wp-block {
		max-width: 800px;
	}
</style>
	<?php
	endif;
}