Probá algo así:
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=iso-8859-1" />
<title></title>
<script>
var w=425;
var h=344;
var em='<object width="{w}" height="{h}"><param name="movie" value="http://www.youtube.com/v/GUj829kvjck&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/GUj829kvjck&hl=es_ES&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="{w}" height="{h}"></embed></object>';
function setMedida(w,h){
window.w=w;
window.h=h;
var r=em.replace(/\{[^\}]*\}/g, function(key){
return window[key.slice(1,-1)];
});
document.getElementById('video').innerHTML=r;
}
onload=function(){
setMedida(425,344);
}
</script>
</head>
<body>
<div id="video"></div>
<form>
<input type="button" value="+" onclick="setMedida(w*1.1,h*1.1)" />
<input type="button" name="Submit" value="-" onclick="setMedida(w/1.1,h/1.1)" />
</form>
</body>
</html>
Ah, veo que se me adelantaron, ja, ja.