change-divi-blog-thumb-size.css

.blog-page .et_pb_post a img, body.archive .et_pb_post a img {
float: left;
margin-top: 0.6em;
margin-right: 1.2em;
border: solid 1px #ccc;
padding: 3px;
}

change-divi-blog-thumb-size.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');