1. Go to Appearance – Theme Options
  2. Navigate to Entry Settings
  3. Be sure to check Enable first full post and the rest excerpt
  4. Next, check Enable Post Thumbnail
  5. Setup Width and Height for featured thumbnail size
  6. Click Save Settings

Recommended Featured Image size is 300px (width) x 380px (height) and above

Once you updated the Featured Image size, make sure to Regenerate Thumbnails sizes.