Adjusting size of page and post featured image

Hello,

I would love to adjust the featured image on all of of my pages and posts as it is on the main page of the blog. I mean that the image covers all the screen.

I tried already to adapt the CSS code in the customizer, but the result was not acceptable and for sure not as perfect as how it is done by you. I guess I either adapted the wrong classes or I used the wrong attributes.

I hope you can help me with my problem.

P.S.: I love your theme!