Changing featured image size

I am trying to make use of featured image on PAGES on my site.

I am basically trying to add a full-width header style to all my posts - so I want to upload images at say 2000x200.

However, Sparkling automatically resizes them to 1140x200.

I can adjust the CSS to ensure that the image appears full width, but it is then of course distorted and not of the quality I uploaded at 2000x200.

What files do I need to edit to stop the 1140 restriction?

Hey there

You need to install this plugin temporary to get the names and locations of the files used for rendering pages:
https://wordpress.org/plugins/say-what/