At its height c 2900 BC , Uruk probably had 50,000 â `` 80,000 residents living in 6 km2 of walled area ; the largest city in the world at the time . At its height c 2900 BC , Uruk probably had 50,000 â `` 80,000 residents living in 6 km2 of walled area ; making it the largest city in the world at the time .