How do I create an alternate RSS feed for tags with custom image sizes for MailChimp?

Filter 'the_content_feed' or 'the_excerpt_rss' and use a thumbnail image of your choice.

Sample code, not tested:

add_filter( 'the_content_feed', 'wpse_70249_custom_thumb' );
add_filter( 'the_excerpt_rss', 'wpse_70249_custom_thumb' );

function wpse_70249_custom_thumb( $content )
{
    global $post;

    $thumb_size="post-thumbnail";

    if ( in_category( 'orange', $post )
    {
        $thumb_size = array ( 500, 500 );
    }
    if ( in_category( 'apple', $post )
    {
        $thumb_size = array ( 100, 100 );
    }

    $thumb = get_the_post_thumbnail( null, $thumb_size );

    return $thumb . $content;
}