I've two feeds, one for articles and something for news records. I have written a function which includes the thumbnail within the feeds. For the reason that function I select which thumb size to exhibit.

However , for that news feed, I would like one image size, and the other image size for that article feed.

Can there be anyway to change the add_filter hook to simply apply the function to 1 category? After which maybe duplicate the function, alter the thumb size and also the category?

Function:

    function insertThumbnailRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) ){
$content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail' ) . '' . $content;
}
return $content;
}

add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');

Tyty!

Possibly try:

function insertThumbnailRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) && in_category(4, $post-ID) ){
    $content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail' ) . '' . $content;
}

if ( has_post_thumbnail( $post->ID ) && in_category(5, $post-ID) ){
    $content = '' . get_the_post_thumbnail( $post->ID, 'different-size' ) . '' . $content;
}

return $content;
}

add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');

Have a look in the codex page for in_category().