Changing post thumbnail size?

My featured images thumbnail size seems to be set at 268 x 268, which I believe is a standard size. However, I’d like my thumbnails to have size 268 x 134. Where do I change this?

I’ve been experimenting with editing the functions.php file, adding the line

set_post_thumbnail_size(268, 134, true );

or changing add_image_size('post-image-mansory', 268, 268, true); to

add_image_size('post-image-masonry', 268, 138, true);

(is “mansory” in the original a typo?) However, none of them affect the thumbnail size. Is there any way of changing the post thumbnail size in Shprink One?

Thanks!