Final answer:
The Byzantine Empire reached its greatest size under Emperor Justinian I.
Step-by-step explanation:
The Byzantine Empire reached its greatest size under Emperor Justinian I, who reigned from 527 to 565. During his reign, the empire encompassed the Balkans to the north, Egypt and parts of North Africa to the south, Anatolia and the Levant to the east, and Italy and the southern Iberian Peninsula to the west.