Show (print) featured image all dimensions (height, width)

Add this function to your functions.php file:

function show($size = NULL) {
    global $post;
    $post_thumbnail_id = get_post_thumbnail_id( $post->ID );
    if(isset($size)){
        $image_data_array = wp_get_attachment_image_src($post_thumbnail_id,$size);
    }else{
        $image_data_array = wp_get_attachment_image_src($post_thumbnail_id);//Default value: 'thumbnail'
    }
    echo $image_data_array[1]."x".$image_data_array[2];//here you can add more chars of strings if you want
}

and use it like this:

<a class="classname" href="https://wordpress.stackexchange.com/questions/256604/<?php the_post_thumbnail_url("full' ); ?>">Full Image <?php show('full'); ?></a>

this is outputting WidthxHeight, you can use any default image size thumbnail, medium, medium_large, large, full or any custom size you have added using add_image_size()