Changing the default thumbnail size

There’s no option that lets you easily do this, although it’s not too hard the fpw_featured_image filter and a custom style.

I’m not in a place where I can actually test this code right now, so use it at your own risk, but I think the following two snippets should do what you need.

1. In your theme’s functions.php file, place the following:

// create custom image size, 80x80 with hard crop
add_image_size( 'fpw_custom_thumbnail, 80, 80, true);

// override the featured image with custom size image
function fpw_custom_featured_image( $featured_image, $featured_page_id ) {
    return get_the_post_thumbnail( $featured_page_id, 'fpw_custom_thumbnail' );
}
add_filter( 'fpw_featured_image', 'fpw_custom_featured_image', 10, 2 );

Note: If your “thumbnail” image size set in Settings > Media is already 80×80, you can get rid of the add_image_size() line and replace ‘fpw_custom_thumnail’ with ‘thumbnail’ in the line beginning with return.

2. In your theme’s style.css file, place the following:

.fpw-layout-wrapped .fpw-featured-image {
    width: 80px; /* overrides width: 45%; */
}

Let me know if this works.

As a final update, I just tested the code. There was a missing quotation mark in:

add_image_size( 'fpw_custom_thumbnail', 80, 80, true);

Otherwise, everything else should work as stated. As mentioned on the plugin’s main page, you’ll probably have to run an image resizing plugin after the change to get it working on images that are already in the media library.