Modify the default recipe image size in Cookbook.

cookbook-image-size.php

<?php

add_filter( 'cookbook_recipe_image_size', 'prefix_get_recipe_image_size' );
/**
 * Modify the default cookbook recipe image size.
 *
 * @since  1.0.0
 * @param  string $size The current image size.
 * @return string
 */
function prefix_get_recipe_image_size( $size ) {
	return 'thumbnail';
}