Increase the width of Post area

Assuming you already have a child theme installed and activated.
https://codex.www.ads-software.com/Child_Themes

In child theme style.css

@media screen and (min-width: 960px) {
	.entry-summary { width: 71.1111%; padding-left:0; }
	/* ----- 71.1111% of 900px is 640px ----- */
}

In child theme functions.php

$content_width = 640;

function my_image_size() {
	add_image_size( 'bold-headline-image-post', 640, 1200 );
}
add_action( 'after_setup_theme', 'my_image_size', 11 );