Browser Resize for Chrome
More About Browser Resize
Options to edit widths and keep popup open after resize (closes by default of maximise).
Nothing malicious in here, unlike in certain other extensions.
Code is on Github https://github.com/grahamlicence/chrome-resize and there is plenty of documentation from Google on how to make your own window resizer.
0.3.0 - Changed window position left and height to stay the same when resizing. Options page added for changing width settings.