function fmd_image_sizes() {
	if ( function_exists( 'add_image_size' ) ) {
		add_image_size( 'bigger_thumbnail', 300, 300, true ); //will be visible in admin
		add_image_size( 'carousel', 1300, 380, true ); //will not be visible in admin
	}
}
add_action( 'init', 'fmd_image_sizes' );

function fmd_chooseable_image_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'bigger_thumbnail' => __('Thumbnail, bigger'),
    ) );
}
add_filter( 'image_size_names_choose', 'fmd_chooseable_image_sizes' );