Code below doesn't change LinePen.Width on the chart with Utils.UseCaches = true, which is the default option. Looks like ChartPen.Width is not being cached correctly.

    public Form1()
    {
      InitializeComponent();
      InitializeChart();
    }

    private Steema.TeeChart.Styles.Line line1;

    private void InitializeChart()
    {
      //Steema.TeeChart.Utils.UseCaches = false;

      tChart1.Dock = DockStyle.Fill;
      tChart1.Aspect.View3D = false;

      line1 = new Steema.TeeChart.Styles.Line(tChart1.Chart);
      line1.FillSampleValues();

      tChart1.Resize += tChart1_Resize;
    }

    void tChart1_Resize(object sender, EventArgs e)
    {
      int w = tChart1.Width;
      int h = tChart1.Height;

      if ((w > 500) && (h > 400))
      {
        line1.LinePen.Width = 5;
      }
      else
      {
        line1.LinePen.Width = 2;
      }
    }