Remove or truncate site rss in

You may achieve this by adding a code snippet to truncate get_the_content_feed via filter:

function truncateFeedContent( $content )
{
    $content = get_the_excerpt();

    return $content;
}
add_filter( 'the_content_feed', 'truncateFeedContent' );

By default, get_the_content_feed show all content