function fGetFileSize(sFileName : String) : Integer;
var fFile: file of Byte;
begin
     Result := 0;
     if FileExists(sFileName) then
     begin
          AssignFile(fFile,sFileName);
          Reset(fFile);
          Result := FileSize(fFile);
          CloseFile(fFile);
     end;
end;

Var
  F : File Of Size;
  Size : Integer;
begin
  if OpenDialog1.Execute then
    begin
      AssignFile(F, OpenDialog1.FileName);
      Reset(F);
      Size := FileSize(F);
      ShowMessage('檔案的大小是﹕'   IntToStr(Size)   'bytes');
      //將以上紅色部分改為如下﹐可求出KB單位
      //InttoStr(Round(Size / 1024))
    end;
end;