Radcliffe – How to change the size of the featured image

Hello, first of all thanks for the theme, I find it really useful.

I have a question, is there any way to change the size of the featured image so it fits the banner of the post.

For example in the example page of the theme there is a post that says Hello World! and I would like that the image is just the size of the Hello World! banner in the main page.

I already tried changing the size of the featured image but I realized that is a theme thing, independent of the image that I upload.

Thanks again!