Display cropped thumbnail not featured image

If I understand correctly, you can try with this code.

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 252, 252, true);

//enter code here

if ( has_post_thumbnail()):

      the_post_thumbnail();

endif; 

OR

if ( has_post_thumbnail()):

      the_post_thumbnail(array(252, 252));

endif;