1.先呼叫以下方法：

          [DllImport("user32.dll", EntryPoint = "SendMessageA")]
          private static extern int SendMessage(IntPtr hwnd, int wMsg, IntPtr wParam, string lParam);

2.設定RichTextBox：

            this.richTextBox1 = new RichTextBox();          
            this.richTextBox1.Text = “contentcontentcontentcontentcontentcontentcontentcontentcontent”;
            this.richTextBox1.Width = this.pPanel.Width-15;
            this.richTextBox1.ScrollBars = RichTextBoxScrollBars.None;
            this.richTextBox1.Location = new Point(0, 0 + this.lab1.Height+10);
            this.richTextBox1.Anchor = (AnchorStyles.Right | AnchorStyles.Left | AnchorStyles.Top);
            //得到RichTextBox高度
            int EM_GETLINECOUNT = 0x00BA;//獲取總行數的訊息號

        void richTextBox1_Resize(object sender, EventArgs e)
        {
            int EM_GETLINECOUNT = 0x00BA;//獲取總行數的訊息號
            int lc = SendMessage(this.richTextBox1.Handle, EM_GETLINECOUNT, IntPtr.Zero, "");
            int sf = this.richTextBox1.Font.Height * (lc + 1) + this.richTextBox1.Location.Y;
            this.richTextBox1.Height = sf;
        }