Margaret,
What you want to do is possible, and relatively easy too!
Here is what I did to achieve what you want to do.
In the HTML editor of your choice, create a small, blank HTML file. In the body tag of the HTML page place your javascript window code.
Code:
<body onload="javascript:window.open('http://www.yahoo.com','bigPic','width=640,height=550')">
Save the file.
In Opus create a browser object and set it to initially hidden. Also make sure it doesn't connect automatically when the page loads. I also unchecked the Outline border and status bar checkboxes. I make this browser object 1x1. Next I created a button with a couple of actions.
The first action is to show the browser. The second action navigates to the HTML file that acts as a launcher. You could also set up variables to contain the names of various launcher pages and then use the Navigate Browser to Variable action. Then I put in a Delay action to delay .05. This gives time for the browser to do its thing. I follow that up with a Hide action on the browser. I now have an external browser window sized to the size I specify and has gone to the location I want.