See here:

1. You can’t resize a window or tab that wasn’t created by window.open.
2. You can’t resize a window or tab when it’s in a window with more than one tab.
3. Also, even if you create window by window.open(…) it is not resizable by default …see 4.
4. To make window created by window.open() resizable, you must open it with resizable feature

myExternalWindow = window.open("http://example.com", "myWindowName", "resizable");
myExternalWindow.resizeTo(500,500); //resize window to 500x500