what could have been the max size of the Roman empire?

Humor aside, even with its vast reach, Rome’s strategy of endless expansion came to a halt in the 2nd century.

The more sensible Romans were not foolish; they grasped the dangers of overextending an empire.

Though Emperor Trajan is celebrated for pushing Rome to its maximum territorial limits, his successor, Hadrian, intentionally relinquished many of these territories, initiated the construction of frontier defenses, and shifted his focus to domestic, civic, and spiritual matters, including public infrastructure, cultural enrichment, and temple construction.

Moderation may bruise the pride of an empire.

Hadrian faced unpopularity among the Roman aristocracy for these decisions, yet history reveals that Rome could not have continued its expansion without igniting the still-dormant flames that would roar in the 3rd century and eventually erase much of Rome’s achievements by the 5th, transforming it into the “Byzantine Empire.” This smaller, less formidable successor was a prolonged and equally intriguing phase of Roman imperial history.

After Trajan, Caracalla too led campaigns into Persia, but Caracalla never invaded Persian mainland, just some miles into the frontier and did not achieve any major conquest there.

Persia was in a civil war at this time, so it was the easiest time for Rome to conquer it if there were any intentions for it.

50 years after that, Emperor Valerian decided to lead an army into Persia, and ended up getting humiliated by the Persians, himself getting captured, which was an insult to rime itself.

This was the last attempt, ever made by a roman emperor to expand, after this, Rome went on a decline from which it could not recover even with the likes of Aurelian, Diocletian and Constantine.

Though during Justinian’s reign, the eastern Rome had great expansion but those were mostly attempts to recover lost lands.

Therefore, we can pretty surely say that Rome had already expanded to the max size it could.

Expanding any further would have meant Rome’s destruction, both economically, and politically due to revolts.

Even Rome having hand of all those territories for centuries, in itself is a great mark, which has not often seen in history.

The best roman emperor, Augustus foresaw this, and strictly ended Roman expansion, only exception being Claudius who conquered Britannia.