Here’s what I would do right now:
1. Create a new image size in WP:
add_action( 'after_setup_theme', function() {
add_image_size( 'your-size-name', 500, 500, true );
} );true tells it to hard crop the image.
2. Regenerate your thumbnails so this size exists using a plugin like this: https://wordpress.org/plugins/regenerate-thumbnails/
3. Tell WPSP to use this new size:
add_filter( 'wpsp_default_image_size', function() {
return 'your-size-name';
} );Adding PHP: https://docs.wpshowposts.com/article/adding-php/