Undo QMainwindow fixed size

I have created a main window using qt designer and set it as fixed size with min and max size values. I now realize I don't want it set as fixed and would like to set it back to the default setting when you first create a central widget. What property items should I change to return to the default state? I have tried changing some items to expandable and setting width and height to zero but the result is a very tiny mainwindow box.

Don't change anything other then resetting the min/max sizes.
The respective properties have a small button that resets values to the defaults

Cheers,
_