A very crude WordPress plugin to search through imported content and replace full size images with a resized counterpart.

bulk-resize.php

<?php

/*
Plugin Name: Bulk Attachment Resize
Description: Destructive operations on DB to replace full images with their resized version.
Author: Agency
Version: 1.0.0
Author URI: http://agency.sc
*/


if ($_GET['dev']) {
	add_action('template_redirect', 'myoutput');
	function myoutput() {
	    
	    $args = array(
	    	'post_type' => 'post',
	    	'post_status' => 'publish',
	    	'posts_per_page' => '1'
	    );

		$the_query = new WP_Query( $args );

		if ($the_query->have_posts()) {
			$i = 0;

			while ($the_query->have_posts()) {
				$the_query->the_post();
				$content = get_the_content();
				
				preg_match_all( '/<img[^>]+src\s*=\s*["\']?([^"\' ]+)[^>]*>/', $content, $matches );

				foreach ($matches[1] as $match) {
					
					//print $match;
					$id = get_image_id($match);

					$large = wp_get_attachment_image_src($id, 'large');
					$content = str_replace($match, $large[0], $content);

				}

				echo $content;

				$i++;
			}
		}
		wp_reset_postdata();

	    exit;
	}
}

function get_image_id($image_url) {

	global $wpdb;
	$prefix = $wpdb->prefix;
	$attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM " . $prefix . "posts" . " WHERE guid='%s';", $image_url )); 
        return $attachment[0]; 

}