I have a template for CPT, where I want to show first post in the loop differently from rest of the post (I need full size image for just the first post in the loop.
I am able to add a CSS class to only the first post in the loop, but it’s not enough.
You can copy index.php to archive-<cpt>.php and modify it as you need to (eg. adding a PHP counter and then a conditional to show something different for the first post)