What do you mean by 'when the empire was at its height'? In terms of territory it was at its strongest during the reign of Justinian (527-565), but we can also argue that it was at its peak under the earlier Macedonian emperors in the ninth/tenth centuries. Given how profoundly the empire changed in the seventh century, the answer will be very different depending on your focus, though of course Constantinople was a fabulously wealthy city in both periods.