Fontsize for the elements of RadTreeView

How to set fontsize for the elements/child nodes of tree of RadTreeView ?

Below is my sample code: (setting as in below code is not working)

<telerik:RadTreeView  x:Name="treeview"
                           ItemTemplate="{StaticResource xx}"
                           ItemsSource="{Binding yy}"
                           ItemsIndent="0">
            <telerik:RadTreeView.ItemContainerStyle>

               <Style BasedOn="{StaticResource xy}" TargetType="telerik:RadTreeViewItem">
                 <Setter Property="IsExpanded" Value="{Binding ab, Mode=TwoWay}" />

                 <Setter Property="FontSize" Value="20" />