1. Go to Theme Options – Entry settings
  2. Scroll down and check Use excerpt summary post and Enable featured thumbnail
  3. Setup Width and Height for featured thumbnail size
  4. Click Save settings

Recommended Featured Image size is 760px (width) x 400px (height) and above

Once you updated the Featured Image size, kindly install Force Regenerate Thumbnails plugin to regenerate your Featured Image size. Go to Tools – Force Regenerate Thumbnail to regenerate all your thumbnail sizes.