Change Featured Image Thumbnail Dimensions

I noticed that when I select an image to be a featured image it crops the selection to 1170 x 545 – actually calling an image with the link appended to be “1170×545.jpg” If I wanted a different image dimension for the Featured Image to appear at (1024 x 683) where would I change it?

I tried adjusting the Post Thumbnail image tag dimensions in the zillah_post_thumbnail() function of functions.php but saw no effect.

Thanks!