Просто нужно настроить сгенерированный код, чтобы он дал вам более определенные значения. Попробуйте это.
<Canvas x:Name="Layer_1" Width="16" Height="16" VerticalAlignment="Center" HorizontalAlignment="Center">
<Ellipse x:Name="_circle" Width="16" Height="16" Stretch="Fill" Fill="#FF666666" HorizontalAlignment="Center" VerticalAlignment="Center" />
<Path x:Name="_darkShadow" Width="16" Height="16" StrokeThickness="3" Stroke="Black" Data="M 2.02083,8.03473L 13.7501,8.03473M 7.84484,2.14584L 7.84484,13.8333" HorizontalAlignment="Center" VerticalAlignment="Center" UseLayoutRounding="False" Canvas.Left="-0.5" Canvas.Top="-0.5"/>
<Path x:Name="_lightShadow" Width="16" Height="16" StrokeThickness="3" Stroke="#FFABABAB" Data="M 2.02083,8.03473L 13.7501,8.03473M 7.84484,2.14584L 7.84484,13.8333" Canvas.Left="-0.5" Canvas.Top="-0.5" Visibility="Collapsed"/>
<Path x:Name="_plus" Width="14" Height="14" StrokeThickness="3" Stroke="#FFFFFFFF" Data="M 2.02083,8.03473L 13.7501,8.03473M 7.84484,2.14584L 7.84484,13.8333" HorizontalAlignment="Center" VerticalAlignment="Center"/>
</Canvas>