the size
property will give you the current size of your widget:
http://qt-project.org/doc/qt-4.8/qwidget.html#size-prop
Alternatively, you could subclass QGraphicsView and re-implement the resizeEvent
:
http://qt-project.org/doc/qt-4.8/qwidget.html#resizeEvent
For a full example, have a look into: