You can use window.resizeTo(numWidth, numHeight) Code (markup): but user can disable this functionality. So there is no way to guarantee, that window will be resized.
FF allows user to disable this function or to open new windows in new tabs. If you use FF for a while I think you saw a yellow message saying that FF blocks the popup window. But you can try to open new window as a reaction on some user action (for example in onclick event).