Schaff, Philip. History of the Christian Church (The Complete Eight Volumes in One). Amazon Kindle Edition, 2014.
Volume 2, Ante-Nicene Christianity A.D. 100-325, “Chapter 4. Organization and Discipline of the Church” Loc. 13792-14769 (part 8).
Schaff begins this section with a survey of literature about the earliest church councils (Schaff 2014, Loc. 14473). Some were more or less ecumenical. Some had much tighter parameters, sometimes applying only to a particular province or region. Public synodical meetings were fairly common and well attended, especially in times of controversy (Schaff 2014, Loc. 14490). After 325 councils were generally closed to people other than bishops. Schaff also notes the bishops acted as successors of apostolic authority rather than as representatives of a congregation (Schaff 2014, Loc. 14499). A lead bishop ascending to visible primacy was not unexpected. However, Schaff does not think this could happen until the empire would allow considerable freedom for Christian practice (Schaff 2014, Loc. 14507). This happened in 325 in conjunction with the Council of Nicea.