Triggering A Pop-Up Window?

I am trying to make a 500 x 500 window pop-up when someone clicks a link for a coupon. But I can't seem to control the size regardless of the values. Here is the function:

HTML Code:
function popupWinCoupon(url, width, height) {
popup = window.open(url,"popup","width=500","height=500");
return false;
}

I also have this at the point of the anchor tag where it is clicked:

HTML Code:
onclick="return popupWinCoupon(this.href,500,500)"
Unfortunately the window repeatedly opens up at about 350px wide and 700px tall.