Filter RSS post thumbnail size

filter-rss-post-thumbnail-size.php

add_filter('seopress_rss_post_thumb_size', 'sp_rss_post_thumb_size');
function sp_rss_post_thumb_size($size) {
    //Will be used with wp_get_attachment_image_src() https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/
    
    $size = 'thumbnail'; //Accepts any registered image size name (thumbnail, medium, large, full or custom size name), or an array of width and height values in pixels (in that order).
    return $size;
}