I have the same question!
I would like to be able to edit the size of the main banner, but I can’t figure out where the “post-thumbnail” dimensions are defined.
I tried changing the dimensions listed in the functions.php file for
add_image_size( 'twentyfourteen-full-width', 1120, 576, true );
But that did not work.
And I don’t know why if the child theme template for the page calls for
<?php twentyfourteen_post_thumbnail(); ?>
how it is out putting the image and what is defining the size of the image, because the page is spitting out:
<div class="post-thumbnail">
<img src="http://visimaxtechnologies.com/wp-content/uploads/2014/09/CoatingProducts-1038x100.jpg" class="attachment-twentyfourteen-full-width wp-post-image" alt="CoatingProducts-1038x100" height="230" width="1038"> </div>
I am working on a site that I inherited, so I did not implement this template and am lost as how to size the featured image attached to the page to show at the size that I want.
Any an all help is appreciated!