divi-custom-thumbnail-sizes.php

<?php

/**
 * Set blog, index, archive & post thumbnail sizes
 */
/* Blog */
function wpt_blog_thumbnail_width( $width ) {
	return 150; //blog thumbnail width in pixels
}
add_filter( 'et_pb_blog_image_width', 'wpt_blog_thumbnail_width');

function wpt_blog_thumbnail_height( $height ) {
	return 150; //blog thumbnail height in pixels
}
add_filter( 'et_pb_blog_image_height', 'wpt_blog_thumbnail_height');

/* Index & archive */
function wpt_index_thumbnail_width( $width ) {
	if( !is_single() ) {
	 	return 150; //index thumbnail width in pixels
	} else {
		return $width;
	}
}
add_filter( 'et_pb_index_blog_image_width', 'wpt_index_thumbnail_width');

function wpt_index_thumbnail_height( $height ) {
	if( !is_single() ) {
		return 150; //index thumbnail height in pixels
	} else {
		return $height;
	}
}
add_filter( 'et_pb_index_blog_image_height', 'wpt_index_thumbnail_height');

/* Post */
function wpt_post_thumbnail_width( $width ) {
	if( is_single() ) {
	 	return 300; //post thumbnail width in pixels
	} else {
		return $width;
	}
}
add_filter( 'et_pb_index_blog_image_width', 'wpt_post_thumbnail_width');

function wpt_post_thumbnail_height( $height ) {
	if( is_single() ) {
		return 300; //post thumbnail height in pixels
	} else {
		return $height;
	}
}
add_filter( 'et_pb_index_blog_image_height', 'wpt_post_thumbnail_height');

?>