Unfortunately, Divi has not made it very easy to customize the portfolio layout in a masonry grid like we've done with Showy (yet!), so the way we've developed the theme is very much a custom solution at this point.
While the masonry grid is a bit "locked" in terms of layout, there is a fairly easy way to adjust the look to your specifications by going to Appearance > Editor > functions.php and searching for the following function:
$sizes = array('one-fourth','one-half','one-fourth','one-fourth','one-fourth','one-fourth','two-fourth','two-half','two-fourth','one-fourth');
You'll see that one-fourth, one-half, one-fourth, one-fourth mimics the first row of the homepage masonry grid layout. All the proceeding values imitate the rest of the rows and columns. All of these sizes are already defined elsewhere in the theme, but to build your own masonry layout design, you just need to follow this example and play around with the order to create your own custom masonry grid layout.