从带有棱镜6的Wpf框架装订高度/宽度

I have a Frame inside my ViewMain.xaml which I need to read the height/width from it. In the ViewModel I've got 2 prism properties and a command that should get called thru a ObservesProperty. The command does some calculations with the height/width. I tried to bind the height/width with all the Modes (TwoWay etc.).I have a Frame inside my ViewMain.xaml which I