[delphi.com.ar]

Yo en Delphi uso esto, habría que traducirlo a C++

Código Delphi [-]
function FileSize(const FileName: string): Int64;
var
  sr: TSearchRec;
begin
  if FindFirst(FileName, faAnyFile, sr) = 0 then
    try
      Result := Int64(sr.FindData.nFileSizeHigh) shl Int64(32) +
                Int64(sr.FindData.nFileSizeLow);
    finally
      SysUtils.FindClose(sr);
    end
  else
    Result := -1;
end;