By default, Easy Image Gallery uses the thumbnail image size for the thumbnail images in the gallery. Here’s how you can specify a different thumbnail size for each post type. For example, you could use one size for Posts, and a completely different size for Pages.
Copy and paste the following function below (mind the opening PHP tag) into your child theme’s functions.php or custom plugin, and adjust the values as required for each post type. You can also easily add your own custom post types if you have any. This code required Easy Image Gallery v1.0.5 or newer.