When does QGraphicsWidget::size() become valid?

[Asperamanca]

I have a custom QGraphicsWidget within a parent QGraphicsWidget, but not in a layout.

To position my child graphicsWidget, I need to take it's size into account.

When is the proper time in the code of my parent QGraphicsWidget to update the child graphicsWidget's position?

I can't do it in the constructor, the size isn't yet valid then.
I could connect to the geometryChanged() signal of the child graphicsWidget, but would also get position changes, not only size changes. This is a bit tricky since I am going to change the position.
The first time I paint is too late, it would flicker.

Other suggestions?