For the Featured Image to be displayed fully, you would need to upload it with the following dimensions:
Recommended featured image width is 1200.
Recommended featured image height is at least 460.
Because our themes are responsive, the images may change depending on the screen size you’re on.
If you can reach out via https://wordpress.com/help/contact, we can look if there are other options so you can display your posts the way you want to.