function ventana(imagename, ancho, alto){
ancho=ancho+20
alto=alto+30
if (ancho > 790) {ancho=790;};
if (alto > 550) {alto=500;};
window.open(imagename,"Imagen","menubar=no,scrollbars=yes,status=no,width="+ancho+",height="+alto+", top=0, left=0, screenX=0, screenY=0")
}


 
<!-- Begin
function lista(form){
var URL = document.form.site.options[document.form.site.selectedIndex].value;
window.location.href = URL;
}
// End -->