customize WPF Reportviewer

Hi there

I want to add a combobox into the Reportviewer Toolbar so that i can change the size (A0,A1,A2 ....) from the Report which is displayed.

Until now i have added the following into the Telerik.ReportViewer.Wpf.xaml

<telerikInput:RadComboBox x:Name="PapersizeCb" 
                          ItemsSource="{Binding PaperSizeList}" 
                          SelectedItem="{Binding SelectedPaperSize}">
</telerikInput:RadComboBox>

My Problem is that i don't know how i can modify the Viewmodel of the Reportviewer to bind my Properties.

The version of Telerik I'm using is Q3 2014 SP1.

 Regards