Divi - custom featured post image size

functions.php

<?php

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 260; /* 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' );