custom-gallery-setting.js

/**
 * Custom Gallery Setting
 */
( function( $ ) {
	var media = wp.media;

	// Wrap the render() function to append controls
	media.view.Settings.Gallery = media.view.Settings.Gallery.extend({
		render: function() {
			media.view.Settings.prototype.render.apply( this, arguments );

			// Append the custom template
			this.$el.append( media.template( 'custom-gallery-setting' ) );

			// Save the setting
			media.gallery.defaults.size = 'thumbnail';
			this.update.apply( this, ['size'] );
			return this;
		}
	} );
} )( jQuery );

custom-gallery-setting.php

<?php
/**
 * Plugin Name: Custom Gallery Setting
 * Version: 1.0
 * Description: Adds a custom setting to the gallery edit in the media modal
 * Author: Dominik Schilling
 * Author URI: http://wphelper.de/
 * Plugin URI: http://wpgrafie.de/
 *
 * License: GPLv2 or later
 *
 *	Copyright (C) 2013 Dominik Schilling
 *
 *	This program is free software; you can redistribute it and/or
 *	modify it under the terms of the GNU General Public License
 *	as published by the Free Software Foundation; either version 2
 *	of the License, or (at your option) any later version.
 *
 *	This program is distributed in the hope that it will be useful,
 *	but WITHOUT ANY WARRANTY; without even the implied warranty of
 *	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *	GNU General Public License for more details.
 *
 *	You should have received a copy of the GNU General Public License
 *	along with this program; if not, write to the Free Software
 *	Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

/**
 * Don't call this file directly.
 */
if ( ! class_exists( 'WP' ) ) {
	die();
}

/**
 * The class.
 */
class Custom_Gallery_Setting {
	/**
	 * Stores the class instance.
	 *
	 * @var Custom_Gallery_Setting
	 */
	private static $instance = null;


	/**
	 * Returns the instance of this class.
	 *
	 * It's a singleton class.
	 *
	 * @return Custom_Gallery_Setting The instance
	 */
	public static function get_instance() {
		if ( ! self::$instance )
			self::$instance = new self;

		return self::$instance;
	}

	/**
	 * Initialises the plugin.
	 */
	public function init_plugin() {
		$this->init_hooks();
	}

	/**
	 * Initialises the WP actions.
	 *  - admin_print_scripts
	 */
	private function init_hooks() {
		add_action( 'wp_enqueue_media', array( $this, 'wp_enqueue_media' ) );
		add_action( 'print_media_templates', array( $this, 'print_media_templates' ) );
	}


	/**
	 * Enqueues the script.
	 */
	public function wp_enqueue_media() {
		if ( ! isset( get_current_screen()->id ) || get_current_screen()->base != 'post' )
			return;

		wp_enqueue_script(
			'custom-gallery-settings',
			plugins_url( 'js/custom-gallery-setting.js', __FILE__ ),
			array( 'media-views' )
		);

	}

	/**
	 * Outputs the view template with the custom setting.
	 */
	function print_media_templates() {
		if ( ! isset( get_current_screen()->id ) || get_current_screen()->base != 'post' )
			return;

		?>
		<script type="text/html" id="tmpl-custom-gallery-setting">
			<label class="setting">
				<span>Size</span>
				<select class="type" name="size" data-setting="size">
					<?php

					$sizes = apply_filters( 'image_size_names_choose', array(
						'thumbnail' => __( 'Thumbnail' ),
						'medium'    => __( 'Medium' ),
						'large'     => __( 'Large' ),
						'full'      => __( 'Full Size' ),
					) );

					foreach ( $sizes as $value => $name ) { ?>
						<option value="<?php echo esc_attr( $value ); ?>" <?php selected( $value, 'thumbnail' ); ?>>
							<?php echo esc_html( $name ); ?>
						</option>
					<?php } ?>
				</select>
			</label>
		</script>
		<?php
	}

}

// Put your hands up...
add_action( 'admin_init', array( Custom_Gallery_Setting::get_instance(), 'init_plugin' ), 20 );