In: themes/divi-child/functions.php

functions.php

<?php
add_action( 'wp_enqueue_scripts', 'my_enqueue_assets' );

function my_enqueue_assets() {

    wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );

}
function mycustom_featured_width() { return 320;} /* Custom featured post image width */
add_filter( 'et_pb_blog_image_width', 'mycustom_featured_width');
function mycustom_featured_height() { return auto;} /* Custom featured post image height */
add_filter( 'et_pb_blog_image_height', 'mycustom_featured_height');
function mycustom_featured_size( $image_sizes ) {
$custom_size = mycustom_featured_width() . 'x' . mycustom_featured_height();
$image_sizes[$custom_size] = 'et-pb-post-main-image-thumbnail';
return $image_sizes;
}
add_filter( 'et_theme_image_sizes', 'mycustom_featured_size' );

?>