https://codex.wordpress.org/Post_Thumbnails aradığınız cevap burada..

// without parameter -> Post Thumbnail (as set by theme using set_post_thumbnail_size())
the_post_thumbnail();

the_post_thumbnail('thumbnail'); // Thumbnail (default 150px x 150px max)
the_post_thumbnail('medium'); // Medium resolution (default 300px x 300px max)
the_post_thumbnail('large'); // Large resolution (default 640px x 640px max)
the_post_thumbnail('full'); // Original image resolution (unmodified)

the_post_thumbnail( array(100,100) ); // Other resolutions