On second thoughts, to be compatible with your version, FileFlush can be easily replaced with FlushFileBuffers API:
Local $hHandle = FileOpen("C:\a.txt"), $tTimer = TimerInit()
Do
ConsoleWrite(FileGetSize("C:\a.txt") & @CRLF)
Sleep(1000)
DllCall("Kernel32.dll", "BOOLEAN", "FlushFileBuffers", "HANDLE", $hHandle)
Until TimerDiff($tTimer) > 20000
FileClose($hHandle)