Change the image size on the attachment page in WordPress

functions.php

<?php
//* Do NOT include the opening php tag

add_filter( 'prepend_attachment', 'custom_prepend_attachment' );
/**
 * Callback for WordPress 'prepend_attachment' filter.
 * 
 * Change the attachment page image size to 'large'
 * 
 * @package WordPress
 * @category Attachment
 * @see wp-includes/post-template.php
 * 
 * @param string $attachment_content the attachment html
 * @return string $attachment_content the attachment html
 */
function custom_prepend_attachment( $attachment_content ){
        
        // set the attachment image size to 'large'
        $attachment_content = sprintf( '<p>%s</p>', wp_get_attachment_link(0, 'large', false) );
        
        // return the attachment content
        return $attachment_content;
        
}