So why do they call him Constantine "the Great"? Well, he did manage to unify a Roman Empire that was threatening to politically disintegrate; he moved the capitol from Rome to Constantinople (a/k/a Byzantium); he triggered an artistic creative spree of buildings and sculptures and mosiacs; and, perhaps most significantly, he legalized Christianity - taking this belief system from an illegal activity punishable by death or imprisonment to an accepted, and even admired, status within the empire.
Interestingly, Constantine did not illegalize the other belief systems in the empire - those versions of polytheistic paganism which had been responsible for executing hundreds of thousands of Christians. Rather, he showed them a type of tolerance which they had never shown to the Christians. Constantine thereby demonstrated what the Romans could expect from their first Christian emperor. Given a chance to exact blood revenge from his former persecutors, he chose not to do so. It is this voluntary surrender of power, the decision not to oppress, which would characterize a new era. Perhaps this is what makes him great.