Changing the size of the image thumbnails ?

Put this into a PHP file (make sure the file has no blank lines at the beginning or end), pop it into the plugins directory, activate the plugin.

Change the “128” to be any number you like, it will be the length (in pixels) of the long side of the generated thumbnail. Note that the thumbnail is generated at upload time, so this won’t change existing thumbnails.

<?php
/*
Plugin Name: Adjust Thumbnail Size
Description: Changes the thumbnail generation size.
Version: 0.1
Author: Otto
Author URI: https://ottodestruct.com/
License: GPL Version 2 - https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
*/
function thumbnail_size_filter($size) {
	return 128;
}
add_filter('wp_thumbnail_max_side_length', 'thumbnail_size_filter');
?>