resizeable windows in pyqt

[Tom Chance]

Hullo,

I know this sounds really daft, but I can't get my windows to be nicely
resizeable in PyQt. I'm using Qt Designer to make the GUI, and I start with
a MainWindow project, without any of the toolbars/menus (so a blank slate).
I then add the various widgets in grids, and when I preview it, the window
resizes but none of the widgets resize with it. I've set the sizePolicy as
"preferred", and played around with other likely looking options, but I
can't work it out! How do I do it?

Thanks,
Tom