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

Recommended Featured Image size is 800px (width) x 600px (height) and above

6- 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.