Making a ‘featured image’ become the image for a full banner (top of page) image in a blog post?

I’m new to WP and I am not quite sure how to approach my goal – I’d like to make featured images in blog posts 1920px wide x 350px high that will become full width banner images at the top of the page.

How do I go about that?

Currently I am using pretty simple .css rules that simply position a background image at the top of the page below the navigation with text written over it – but I’d like that to be applied to all blog posts…

How would you tackle this?

Thanks for all direction…