Code

class RectToStringConverter : IValueConverter
{
  public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
  {
    var bounds = (Rect)value;

    return parameter.ToString() == "W" ? bounds.Width.ToString("#.00") : bounds.Height.ToString("#.00");

  }

  public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
  {
    throw new NotImplementedException();
  }
}

// ...

static RectToStringConverter converter = new RectToStringConverter();

// ...

var bindingWidth = new Binding("Bounds");
bindingWidth.Source = overlayNode;
bindingWidth.Mode = BindingMode.OneWay;
bindingWidth.Converter = converter;
bindingWidth.ConverterParameter = "W";

var bindingHeight = new Binding("Bounds");
bindingHeight.Source = overlayNode;
bindingHeight.Mode = BindingMode.OneWay;
bindingHeight.Converter = converter;
bindingHeight.ConverterParameter = "H";

textBoxW.SetBinding(TextBox.TextProperty, bindingWidth);
textBoxH.SetBinding(TextBox.TextProperty, bindingHeight);