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

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.

In Designer, activate the centralWidget and assign a layout, e.g. horizontal or vertical layout. Then your QFormLayout will automatically resize.

Always make sure, that all widgets have a layout! Otherwise, automatic resizing will break with that widget!

See also

Controls insist on being too large, and won't resize, in QtDesigner