Believing the roman empire had grown too large fora single ruler diocletian?

Yes he did. He created the tetrarcy (rule by four) whose purpose was to improve the deference of the vast frontiers of the Roman Empire, which were under constant attack. He appointed Maximian as his co-emperor. Maximian was in charge of the western part of the empire and Diocletian took charge of the eastern part. These two men were senior emperors (Augusti). The junior emperors (Caesars) subordinate to the Augusti were appointed. They were responsible for the most troubled frontier areas, the river Rhine in the west and the river Danube in the east.