When was Roman army at the height of its power?

[ValentinianVictrix]

You could arguably say that if we are using sheer numbers of people, plus the size of the military forces able to be used, plus the total territory of the Roman Empire , then it might well be the period from when Constantine reigned to the death of Julian was when Rome was at its zenith. Apart from the old province of Dacia, which had been abandoned before the reign of Constantine, the Empire was at its greatest extent during that time frame and stretched from the border of Scotland in Briton to the banks of the Euphrates in the Middle East.