pen类中如何调整箭头的大小

        Pen p2_green = new Pen(Brushes.SeaGreen, 2);
        Pen p2_black = new Pen(Brushes.Black, 2);
        AdjustableArrowCap aac = new AdjustableArrowCap(4, 6);
        p2_green.CustomStartCap = aac;
        p2_black.CustomStartCap = aac;