<?php
add_filter('oembed_result','twitter_no_width',10,3);
// add_filter('embed_oembed_html','twitter_no_width',10,3);
function twitter_no_width($html, $url, $args) {
        if (false !== strpos($url, 'twitter.com')) {
                $html = str_replace('width="550"','width="200"',$html);
        }
        return $html;
}