Allows to add auto resizing to a custom wp_editor() instance added in a metabox.

auto_resize_wp_editor.php

<?php
/* Define the custom box */
add_action( 'add_meta_boxes', 'my_meta_box' );

/* Do something with the data entered */
add_action( 'save_post', 'custom_save_post_meta' );

/* Adds a box to the main column on the Post and Page edit screens */
function my_meta_box() {
	add_meta_box( 'wp_editor_metabox', 'Metabox with Editor', 'wp_editor_meta_box' );
}

/* Prints the box content */
function wp_editor_meta_box( $post ) {

// Use nonce for verification
	wp_nonce_field( plugin_basename( __FILE__ ), 'metabox_nonce' );

	$field_value = get_post_meta( $post->ID, 'custom_meta', true );
	//Add a resizable editor, with a minimum height of 100 px
	$args = array(
		'tinymce'          => array(
			'autoresize_min_height' => 100,
			'wp_autoresize_on'      => true,
			'plugins'               => 'wpautoresize',
			'toolbar1'              => 'bold,italic,underline,link,unlink,forecolor',
			'toolbar2'              => '',
		),
	);
	wp_editor( $field_value, 'custom_meta', $args );
}

/* When the post is saved, saves our custom data */
function custom_save_post_meta( $post_id ) {

// verify if this is an auto save routine.
// If it is our form has not been submitted, so we dont want to do anything
	if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
		return;

// verify this came from the our screen and with proper authorization,
// because save_post can be triggered at other times
	if ( ( isset ( $_POST['metabox_nonce'] ) ) && ( ! wp_verify_nonce( $_POST['metabox_nonce'], plugin_basename( __FILE__ ) ) ) )
		return;

// Check permissions
	if ( ( isset ( $_POST['post_type'] ) ) && ( 'page' == $_POST['post_type'] ) ) {
		if ( ! current_user_can( 'edit_page', $post_id ) ) {
			return;
		}
	}
	else {
		if ( ! current_user_can( 'edit_post', $post_id ) ) {
			return;
		}
	}

// OK, we're authenticated: we need to find and save the data
	if ( isset ( $_POST['custom_meta'] ) ) {
		update_post_meta( $post_id, 'custom_meta', $_POST['custom_meta'] );
	}

}
?>