Can I Resize A Certain Post Type Only

If the upload HTTP request sends a header unique to that post type, or if it comes from a URL unique to the post or post type, then you probably could build such a thing. You’d need to integrate with the imsanity_get_max_width_height filter that is run like so:
apply_filters( ‘imsanity_get_max_width_height’, array( $w, $h ), $source );

More context at https://github.com/nosilver4u/imsanity/blob/e41c8a26c7146bc588c8d4aacb046eb53e714051/imsanity.php#L167