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function MyGetFileSize(const Filename: string): TULargeInteger;
var
  Find: THandle;
  Data: TWin32FindData;
begin
  Result.QuadPart := -1;
  Find := FindFirstFile(PChar(Filename), Data);
  if (Find <> INVALID_HANDLE_VALUE) then
  begin
    Result.LowPart := Data.nFileSizeLow;
    Result.HighPart := Data.nFileSizeHigh;
    Windows.FindClose(Find);
  end;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
  if (OpenDialog1.Execute) then
    ShowMessage(IntToStr( MyGetFileSize(OpenDialog1.FileName).QuadPart ));
end;