http://agathe.gr/democracy/the_ekklesia.html
The Ekklesia (Citizens' Assembly) All Athenian citizens had the right to attend and vote in the Ekklesia, a full popular assembly which met about every 10 days. All decrees (psephismata) were ratified ... The Ekklesia (Citizens' Assembly) All Athenian citizens had the right to attend and vote in the Ekklesia, a full popular assembly which met about every 10 days. ... In this early form the seating capacity was about 5,000. ... In a third and final phase dated to the late 4th century B.C., the seating capacity was greatly increased, to accommodate as many as 13,500 people.