[ecfisa]

Hola.

Otra opción:

void __fastcall TForm1::Button1Click(TObject *Sender)
{
TSearchRec sr;
  if(OpenDialog1->Execute()){
    FindFirst(OpenDialog1->FileName,faArchive, sr);
    ShowMessage(OpenDialog1->FileName+" Size:"+IntToStr(sr.Size /1024)+ "Kb.");
  }
}

Y creo que la traducción de la sugerida por Chris sería:

void __fastcall TForm1::Button2Click(TObject *Sender)
{
 TFileStream *FStream;
 if(OpenDialog1->Execute()) {
  FStream = new TFileStream(OpenDialog1->FileName, fmOpenRead);
  ShowMessage(OpenDialog1->FileName+" Size:"+IntToStr(FStream->Size /1024)+ "Kb.");
  delete FStream;
 }
}

Un saludo.