1- Go to Appearance – Theme Options
2- Scroll down and check Show 2 columns blog listing
3- Select Entry settings tab
4- Scroll down and check Enable excerpt entry and Enable Post Thumbnail
5- Click Save settings

Post Thumbnail size is 760px width x 570px height

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.