How to make a Qt Widget grow with the window size?

Question

I want to have a small QFormLayout that grows to fill its parent widget.

I created a new .ui file using the QWidget template in Qt

Answer 1

The accepted answer (its image) is wrong, at least now in QT5. Instead you should assign a layout to the root object/widget (pointing to the aforementioned image, it should be the MainWindow instead of centralWidget). Also note that you must have at least one QObject created beneath it for this to work. Do this and your ui will become responsive to window resizing.