the_post_thumbnail('thumbnail');    // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium');       // Medium resolution (default 300px x 300px max)
the_post_thumbnail('medium_large'); // Medium Large resolution (default 768px x 0(means automatic height by ratio) max) since WP version 4.4
the_post_thumbnail('large');        // Large resolution (default 640px x 640px max)
the_post_thumbnail('full');         // Original image resolution (unmodified)