1- Go to Appearance – Theme Options
2- Select Entry settings tab
3- Check Enable excerpt entry and Enable Post Thumbnail
4- Setup the Width and Height for the post thumbnail

Recommended Featured Image size is 800px (width) x 500px (height) and above

5- Click Save settings

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.