I want change font size to print text

public override string PrintKOTOrders(string connectionString, long[] orderDetailNumbers, string userName, bool isReprint)
{
    string error = string.Empty;

    RawPrinterHelper receiptPrinter = new RawPrinterHelper();

    ReceiptFunction receiptFunction = new ReceiptFunction();
    

    DataSet ds = receiptFunction.GetKOTNoteFromDataSet(connectionString, orderDetailNumbers);

    DataTable headerTable = new DataTable();
    DataTable foodTable = new DataTable();
    DataTable foodModifierTable = new DataTable();

    DataTable foodPrinters = new DataTable();
    DataTable foodModifierPrinters = new DataTable();
    DataTable reciprtPrinters = new DataTable();

    if (ds.Tables.Count > 0)
    {
        headerTable = ds.Tables[0];

        if (ds.Tables.Count > 1)
        {
            foodTable = ds.Tables[1];

            string[] printerName = { "PrinterName", "FoodDept" };

            foodPrinters = this.GetDistinctRecords(foodTable, printerName);

            if (ds.Tables.Count > 2)
            {
                foodModifierTable = ds.Tables[2];

                foodModifierPrinters = this.GetDistinctRecords(foodModifierTable, printerName);
            }

            reciprtPrinters = foodPrinters.Copy();
            reciprtPrinters.Merge(foodModifierPrinters);

            reciprtPrinters = this.GetDistinctRecords(reciprtPrinters, printerName);

        }
        else
        {
            error = "No Food Records";
        }
    }
    else
    {
        error = "No Header Records";
    }

    string displayString = string.Empty;

    foreach (DataRow printer in reciprtPrinters.Rows)
    {
        receiptPrinter.PrinterName = printer["PrinterName"].ToString();

        if (isReprint)
        {
            this.PrintImage.PrintDuplicateImage(receiptPrinter, 28);
        }
        
        displayString += Environment.NewLine;
        displayString += "Order No   : " + headerTable.Rows[0]["OrderId"].ToString().Trim() + "/" + printer["FoodDept"].ToString().Trim() + Environment.NewLine;
        displayString += "Order Date : " + headerTable.Rows[0]["OrderDate"].ToString() + Environment.NewLine;
        displayString += "Table(s)   : " + headerTable.Rows[0]["TableblNos"].ToString().Trim() + Environment.NewLine;
        displayString += "Res No     : " + headerTable.Rows[0]["ReservationID"].ToString().Trim() + "  No of Guest(s): " + headerTable.Rows[0]["NoOfGuest"].ToString().Trim() + Environment.NewLine;
        displayString += "Order Taken: " + userName + Environment.NewLine;
        displayString += "C  Item                         Quantity" + Environment.NewLine;
        displayString += "----------------------------------------" + Environment.NewLine;
        


        this.Print(receiptPrinter, displayString);
        displayString = string.Empty;

        string foodDepartmentID = printer["FoodDept"].ToString();

        string filter = string.Format("FoodDept = '{0}'", foodDepartmentID);
        DataRow[] foodPrinter = foodTable.Select(filter);

        foreach (DataRow foodRow in foodPrinter)
        {
            this.PrintKOTLines(foodRow, false, receiptPrinter, false);
        }

        filter = string.Empty;
        filter = string.Format("FoodDept = '{0}' AND ModifierString is NOT NULL", foodDepartmentID);

        DataRow[] foodModifierPrinter = foodModifierTable.Select(filter);

        foreach (DataRow foodRow in foodModifierPrinter)
        {
            this.PrintKOTLines(foodRow, false, receiptPrinter, false);

            // modifier print section

            string modifiers = foodRow["ModifierString"].ToString();

            string[] modifierArray = modifiers.Split(',');
            List<string> modifierList = new List<string>(modifierArray);

            var grouped = modifierList.GroupBy(s => s).Select(g => new { FoodID = g.Key.ToString().Trim(), Count = g.Count() });

            foreach (var item in grouped)
            {
                filter = string.Empty;
                filter = string.Format("FoodDept = '{0}' AND ModifierString is NULL AND  FoodID = '{1}'", foodDepartmentID, item.FoodID);

                DataRow[] modifierPrinter = foodModifierTable.Select(filter);

                if (modifierPrinter.Count() > 0)
                {
                    string modifierName = modifierPrinter[0]["NAME"].ToString();
                    int modifierCount = int.Parse(modifierPrinter[0]["Quantity"].ToString());

                    int foodCount = 1;
                    int.TryParse(foodRow["Quantity"].ToString(), out foodCount);
                    int modifierQuantity = 0;

                    if (modifierCount < 0)
                    {
                        modifierQuantity = foodCount * int.Parse(item.Count.ToString()) * modifierCount;
                    }
                    else
                    {
                        modifierQuantity = foodCount * int.Parse(item.Count.ToString());
                    }

                    int length = (31 - modifierName.Length);
                    if (length < 0)
                    {
                        length = 0;
                    }

                    if (modifierName.Length <= 25)////30
                    {
                        if (modifierQuantity.ToString().Length > 1)
                        {
                            displayString += "     " + modifierName + " ".PadLeft(length) + " " + modifierQuantity.ToString() + Environment.NewLine;
                        }
                        else
                        {
                            displayString += "     " + modifierName + " ".PadLeft(length) + "  " + modifierQuantity.ToString() + Environment.NewLine;
                        }
                    }
                    else
                    {
                        int p = 0;
                        while (modifierName.Substring(p, p + 26).Length >= 26)
                        {
                            if (modifierQuantity.ToString().Length > 1)
                            {
                                displayString += "    " + modifierName + " ".PadLeft(length) + "    " + modifierQuantity.ToString() + Environment.NewLine;
                            }
                            else
                            {
                                displayString += "    " + modifierName + " ".PadLeft(length) + "  " + modifierQuantity.ToString() + Environment.NewLine;
                            }

                            modifierName = modifierName.Substring(p + 26, modifierName.Length - 26);

                            if (modifierName.Length < 26)
                            {
                                displayString += ("    " + modifierName + Environment.NewLine);
                                break;
                            }

                        }

                    }

                    //displayStringL1 += "    " + modifierName + " ".PadLeft(length) + modifierQuantity.ToString() + Environment.NewLine;

                    this.Print(receiptPrinter, displayString);
                    displayString = string.Empty;
                }

            }

        }
        displayString += string.Empty + Environment.NewLine + Environment.NewLine;
        displayString = this.CutPrinterPaper(displayString);
        this.Print(receiptPrinter, displayString);
        displayString = string.Empty;
    }

    return error;

}