Passing Vars To Window.open()?

I want to pass window size variables to javascript's window.open() function from a flash movie (actionscript) to javascript embedded in an html page. The idea is to generate a pop-up window when the user clicks a button in the flash movie. I've programmed the flash movie button like this:

on (release) {
getURL("javascript:open_win('http://www.mysite.com', winfeatures)"); }

where winfeatures is a string containing the pop-up window options, e.g. width=400,height=300,resizable=1 . I have 'traced' the string to make sure it is formed properly. These variables may change so I don't want to just insert them verbatim as the arguments. They are read from an xml file and made into a string - e.g.

(var winfeatures:String = "width="+myxml.attributes.width+",height="+myxml.attributes.height);

In the html page I have the following javascript:

<SCRIPT LANGUAGE=JavaScript>
function open_win(URLtoOpen, windowFeatures) {
newWindow=window.open(URLtoOpen, "myWin", windowFeatures);
}
</SCRIPT>

but it's not working. It seems the windowFeatures isn't being read (or read properly) and the pop-up opens with a default size.