Thanks for this question.
We always accept you will use the large image for feature image.
Feature image size should be minimum 1000px larger than the full area will cover.
Our next version will add some options for the small feature image.
Thank you for a great answer, easy to follow. I was cutting the images down to make them load faster. I discovered that Unsplash gives me a choice of sizes, and your answer is the key I needed.