The piazza in front of St. Peter's was built by Bernini between 1657-1667. It was designed with the Feast of Corpus Christi especially in mind, which at the time was very popular and engendered massive public processions. Today it is used for solemn Masses and ceremonies; for canonizations; for the Pope's Sunday angelus, a devotion commemorating the Incarnation; and for the Pope's Wednesday audience, weather permitting.

St. Peter's Square has the shape of an immense ellipse (the visitor who stands in one of the two centers of this ellipse, marked by two white disks, one on each side of the obelisk, sees a single row of columns), 320 m. long and 240 m. wide, at its broadest point. http://saintpetersbasilica.org/Exterior/Square/Square.htm and http://stpetersbasilica.info/Exterior/Colonnades/Saints-List-Colonnades.htm