Algo así debería funcionarte:
Código:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<script>
var embed='<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/MIbsJ8Przco&hl=es_ES&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/MIbsJ8Przco&hl=es_ES&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>';
function resize(e,w,h){
var pat1=/width=["|\']([^"|\']+)["|\']/ig;
var pat2=/height=["|\']([^"|\']+)["|\']/ig;
var ret=e.replace(pat1,'width="'+w+'"');
ret=ret.replace(pat2,'height="'+h+'"');
return ret;
}
onload=function(){document.body.innerHTML=resize(embed,276,155);}
</script>
</head>
<body>
</body>
</html>