A SelectionHandle takes the shape of the AdornedElement that it adorns, but its Stroke, StrokeThickness, Fill, et al. must still be set. You should not set its Width or Height, because those will be determined by the size and shape of the adorned element.

A simple example definition:

              <DataTemplate>
                <go:SelectionHandle Stroke="Red" StrokeThickness="2" />
              </DataTemplate>
            

If a SelectionHandle is not the only element of the selection adornment template, it should be named as the Node.LocationElementName for the whole adornment.

System.Object
   System.Windows.DependencyObject
      System.Windows.UIElement
         System.Windows.FrameworkElement
            System.Windows.Shapes.Shape
               System.Windows.Shapes.Path
                  Northwoods.GoXam.SelectionHandle

Reference