I think I have solved it after a bit of digging so I am sharing so that others will have the fix in one go.
If you enable WP Offload Media with S3 bucket or even Google Cloud Storage GCS bucket then.
1. Check the List settings under IMAGES and make sure width and height is left blank.
2. Add this to your functions.php (not sure how to add it via Elements (if using GP Premium like I am)
/** S3 Images with WP Show Posts **/
add_filter( 'wpsp_default_image_size', function() {
return 'medium';
} );
You can adjust the size to “thumbnail” or “large” or whatever you prefer under the return value.
If there are better ways of doing this please let me know but I do hope this helps someone else setting up WP Offload Media with WP Show Posts.