Use Slider to control the Glow

   
    
<Window x:Class="BitmapEffects.Window4"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Title="Bitmap Effects" Height="538" Width="600"
  >
    <StackPanel>
        <Canvas>
            <Ellipse Height="100" Width="100" Fill="Blue" Stroke="Black"/>
            <Canvas.BitmapEffect>
                <OuterGlowBitmapEffect 
            GlowSize="{Binding Path=Value, ElementName=sliderGlow}"
              />
            </Canvas.BitmapEffect>
        </Canvas>
        <TextBlock>Outer Glow</TextBlock>
        <Slider Minimum="0" Maximum="20" Name="sliderGlow" Value="14"/>
        <TextBox Text="{Binding ElementName=sliderGlow, Path=Value}"/>


    </StackPanel>
</Window>