change font size in system.windows.controls.richtextbox for selection

Question

I'm using a system.windows.controls.richtextbox, so the user can make notes. Now i want the the user to be able to change the font size with a combo box. The problem is, that i can't find a way to change the size of the selected text or, if there is no text selected, the size of the text from the cursor instead of everything.

    <Grid.RowDefinitions>
        <RowDefinition Height="auto"></RowDefinition>
        <RowDefinition Height="*"></RowDefinition>
    </Grid.RowDefinitions>
    <StackPanel
            Foreground="#000000"
            FontWeight="Bold"
            VerticalAlignment="Center"
            Text="{x:Static p:Resources.FontSize}" 
         />
        <ComboBox 
            x:Name="fontsize" 
            Margin="10,2,20,2"
            >
        </ComboBox>
    </StackPanel>
    <RichTextBox
        Name="textbox"
        Grid.Row="1" Grid.ColumnSpan="2" >
    </RichTextBox>
</Grid>



public NotesSetup()
        {
            InitializeComponent();
            this.AddFonts();
            this.fonts.SelectedIndex = 1;
            this.textbox.IsInactiveSelectionHighlightEnabled = true;
            this.fonts.SelectionChanged += (s, a) =>
            {
            if (!this.textbox.Selection.IsEmpty)
            {

            }
            else
            {
                this.textbox.FontFamily = this.fonts.SelectedItem as FontFamily;
            }
        };
        for(int i = 8; i < 80; ++i)
        {
            this.fontsize.Items.Add(i);
        }
        this.fontsize.SelectedItem = 12;
        this.fontsize.SelectionChanged += (s, a) =>
        {
            this.textbox.FontSize = int.Parse(this.fontsize.SelectedItem.ToString());
        };
    }

    private void AddFonts()
    {

        foreach(System.Drawing.FontFamily f in System.Drawing.FontFamily.Families)
        {
            this.fonts.Items.Add(new FontFamily(f.GetName(0)));
        }
    }
}