regular expression for file size in asp.net?

<asp:FileUpload ID="fuFile" runat="server" />

<asp:CustomValidator ID="CustomValidator1" runat="server" ControlToValidate="fuFile"

   ErrorMessage="File size should not be greater than 1 KB." OnServerValidate="CustomValidator1_ServerValidate"></asp:CustomValidator>

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)

    {

        if (fuFile.FileBytes.Length > 1024) // 1024*KB of file size

        {

            args.IsValid = false;

        }

        else

        {

            args.IsValid = true;       

        }

    }