string|int $size, int $thumbnail_id, WP_Post $post )
Filters the size used to display the post thumbnail image in the ‘Featured image’ meta box.
Note: When a theme adds ‘post-thumbnail’ support, a special ‘post-thumbnail’ image size is registered, which differs from the ‘thumbnail’ image size managed via the Settings > Media screen.
Requested image size. Can be any registered image size name, or an array of width and height values in pixels (in that order).
Post thumbnail attachment ID.
The post object associated with the thumbnail.
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$size = apply_filters( 'admin_post_thumbnail_size', $size, $thumbnail_id, $post );