function FormatByteSize(Bytes: Int64): string;
//Converts a numeric value into a string that represents the number expressed as a size value in bytes, kilobytes, megabytes, or gigabytes, depending on the size
var
  iSize: array[0..255] of WideChar;
begin
  Result := '';
  // same formating used in statusbar of Explorer
  Result := ShLwApi.StrFormatByteSizeW(Bytes, iSize, Length(iSize) - 1);
end;

uses ShLwApi, hyieutils;

procedure TForm1.Button1Click(Sender: TObject);
var
 iOpenDialog: TOpenDialog;
 iFilename: string;
 iFileSize: integer // bytes
begin
 iOpenDialog := TOpenDialog.Create(self);
 try
   iOpenDialog.Filter := OPEN_MEDIA_FILES_EXTENDED;
   if iOpenDialog.Execute then 
   begin
     iFilename := iOpenDialog.Filename;
     // get the filesize
     iFileSize := IEGetFileSize(iFilename)
     Label1.Caption := 'File Size: ' + FormatByteSize(iFileSize); 
   end;
  finally
    iOpenDialog.Free;
  end;
end;