This is how:
http://www.w3schools.com/js/js_window_screen.asp.
However, it would be better to avoid popups. In many cases, they are inconvenient (it's better to leave it up to the user, to create a new window, a tab or navigate out of existing content on the same page). Also, due to some massive advertizement using pop-ups, many users may block them using Web browsers' features or plug-ins developed for this purpose.
—SA