[qt4] How to make a Qt Widget grow with the window size?

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 Designer. I put a QFormLayout inside that 'window', then put some controls inside that QFormLayout.

This all works reasonably well, but the QFormLayout always stays at the size I set in Qt Designer. I would like the QFormLayout to fill its parent widget and grow/shrink with it.

How can I accomplish that?

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.

I found it was impossible to assign a layout to the centralwidget until I had added at least one child beneath it. Then I could highlight the tiny icon with the red 'disabled' mark and then click on a layout in the Designer toolbar at top.

You need to change the default layout type of top level QWidget object from Break layout type to other layout types (Vertical Layout, Horizontal Layout, Grid Layout, Form Layout). For example:

To something like this: