Width-Height of control - please help?

In my WPF desktop application, I have two UserControls.  Both have a canvas which contains a chart. I am placing these two UserControls in a StackPanel within another UserControl.

I want the width of the Charts to be same as width of the parent UserControl - which is the width of the main application window.

I also want to set the height the two charts.

Where do I set these? I would prefer it in Code behind - if so, where do I do this?

THanks.