function GetFileSizeInt64(FileName:String):Int64;
var
  FileHandle:integer;
  i,j:Cardinal;
  k:int64;
begin
  FileHandle:=CreateFile(pchar(FileName),
                         GENERIC_READ,
                         FILE_SHARE_READ,
                         nil,
                         OPEN_EXISTING,
                         FILE_ATTRIBUTE_NORMAL,0);
  j:=GetFileSize(FileHandle,@i);
  CloseHandle(FileHandle);
  if i>0 then begin
    k:=i;
    k:=k shl 32;
    k:=k+j;
  end else k:=j;
  Result:=k;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  FileSize:Int64;
begin
  FileSize:=GetFileSizeInt64('C:FileName.Ext');
  ShowMessage(inttostr(FileSize));
end;