Create New RSS Feed for MailChimp When Resizing Thumbnails for Bloglovin

functions.php

<?php

//* Do NOT include the opening php tag shown above. Copy the code shown below.

add_filter( 'send_images_rss_thumbnail_size', 'prefix_change_thumbnail_alternate_feed' );
function prefix_change_thumbnail_alternate_feed( $image_size ) {
    if ( is_feed( 'email' ) ) {
        $image_size = 'mailchimp-rss';
    }
    return $image_size;
}

add_image_size( 'mailchimp-rss', 300, 150, true );