Changing table column size

alter table table_name alter column column_name char(500)

How to restrict textbox to enter 500 characters only after that if we insert any character it will display pop up message

Design page
<asp:TextBox ID="txtreqtitle" runat="server" CssClass="textCopy" Width="260px" Columns="25"
 MaxLength="500" TextMode="MultiLine"  Rows="3" Height="55px"  ></asp:TextBox>

aspx.cs page
this.txtreqtitle.Attributes.Add("onKeyPress", "return textCounter(this, 500);");
this.txtreqtitle.Attributes.Add("onKeyUp", "return textCounter(this, 500);");

javascript on radio button click

 <script type="text/javascript">

function Enable_PromProg() {
            debugger;
            var rdbPromProgYes = document.getElementById("<%=rdbPromProgYes.ClientID%>");
            var rdbPromProgNo = document.getElementById("<%=rdbPromProgNo.ClientID%>");
            var trPromotional = document.getElementById("<%=trPromotional.ClientID%>");
            if (rdbPromProgYes != null) {

                trPromotional.style.display = 'none';
                if (rdbPromProgYes.checked != true && rdbPromProgNo.checked != true) {

                    trPromotional.style.display = 'none';
                    //txtPromotionalDesc.disabled = true;
                }
                else if (rdbPromProgYes.checked == true) {
                    trPromotional.style.display = '';
                    //txtPromotionalDesc.disabled = false;
                }
                else if (rdbPromProgNo.checked == true) {

                    trPromotional.style.display = 'none';
                }
            }
        }


</script>

<asp:RadioButton ID="rdbPromProgYes" runat="server" Text="Yes" GroupName="PromProg"
                                                onclick="javascript:Enable_PromProg();" />

Case statement in SQL

"PaymentReleaseDate"=
Case ISNULL(Convert(Varchar(20),tgr.ap_print_date,101),'') when '' then
( Select milestone_release_date from t_grant_milestones where grant_id= tgr.grant_id and entry_id=tgm.entry_id)
else tgr.ap_print_date 
End AS  PaymentReleaseDate,