Change the size of the featured image on front and search pages

Normally you’d need to create a child theme to override the setup function that sets those sizes in function cubic_setup()

But since Cubic is already a child theme of Boardwalk you won’t be able to do that. You could either fork Cubic or try a grandchild theme plugin:
https://docs.appthemes.com/tutorials/creating-grandchild-themes/

So, there’s no way of doing it through CSS?

Hmm.. how would I do it with Boardwalk? I like some things about Cubic, but I wouldn’t mind simply using Boardwalk if I can change those sizes.

In Cubic you could try something a bit hacky with CSS like this, adjusting the percentage as needed:

.archive .hentry, .blog .hentry, .search-results .hentry {
     padding-bottom: 20% !important;
     width: 20% !important;
}