If you want to change related posts thumbnail size then first you need to create a Custom Plugin and add below the filter.
Example code:
add_filter('ampforwp_related_post_image_size','prefix_related_post');
function prefix_related_post($image_dimensions){
$image_dimensions['width'] = '300'; //Add your width which you need
$image_dimensions['height'] = '300'; //Add your height which you need
return $image_dimensions;
}The Output of change-related posts thumbnail size
That’s it for this tutorial!
We hope it helped you. If you have any feedback or questions, then please share them in the comments section below.