How to change QMainWindow size at runtime?

[myfifth]

Basically, I'd like to build an app with main window by following steps,

1. build main window (size was not determined yet at this moment). now, main window only contained toolbar, menu, status bar, without a central widget
2. then dynamically create the first widget (QFrame-derived), it would be central widget of main window and it would expand main window size according to its size
3. continue to dynamically create some widgets and add them to central widget.

Briefly, main window size depends on central widget size.

Since main window size = all bars size + central widget size,
so resize() didn't work for my case, it actually resized entire geometry area which contains those bars, but I've no idea about what those bars size are.

Hopefully I'm clear for you.
Thanks.