The best known urban gathering spaces, such as the main piazzas of Tuscan towns, share several characteristics that appear to be requirements for successful outdoor urban meeting places anywhere. First in importance is that they are large enough to provide for a variety of activities yet small enough so everyone can recognize everyone else in the piazza.
Then they are shaped to take advantage of that fact, being convex, their walls curving in on themselves, a shape that, again, allows everyone in the space to see everyone else, which is the basis for the many social interactions such piazzas are renowned for.
Most historic plazas limit their longest dimension to 100 meters /110 yds, which is the farthest we can visually distinguish other persons. The same is true for most sports arenas of any vintage, and for the same reason; so all spectators can see everything happening in the space. In other words, our social field of vision is the traditional basis for our most frequented social arenas.
Sound is another human connection made in public spaces, so plazas are also sized to make people’s voices distinguishable. We distinguish sounds other people make from other sounds and instinctively focus on them. We can hear shouts for help at 50 to 70 meters (55 to 75 yards). At 35 meters (38 yards) we can hear the loud conversation from the pulpit or stage. Short messages can be conveyed at 20-25 meters (22/27 yards), but real conversation can only occur within 7 meters (22 ft.). The closer the distance between two talkers below seven meters the more nuanced the conversation and understanding can be.
Historically, larger open spaces have been pared down to optimal sizes in several ways. They have been partially enclosed in buildings by arcades, as in Bologna or along Paris’s Rue de Rivoli. In many places, a large central focal point—obelisk, arch, statue—has effectively created a set of smaller spaces to each side. Or alleés of trees along either side of boulevards like those along the Champs Elysées isolate the wide central roadway and make the side spaces eminently social spaces.
The overall Piazza del Campo in Siena is overlarge at 90 by 135 meters, but it is effectively reduced to a more optimal 100-meter (110-yard) square by an inner ring of large stone bollards. It has the other attributes of great spaces—a slope down toward city hall for orientation and viewing, cafés with outdoor seating around three edges, other retail spaces, The attracted crowds tend to stand by the bollards, window shop along the edges, sit at the cafes, and promenade between the bollards and the café seating. People sit or stand in the center of the plaza during major events.
A lesser example of the role size plays in gathering places comes from my landscape contracting experience. I was asked to make a large square of lawn in front of two flanking apartment buildings attractive to residents for sitting and lounging. Although the lawn had a fine view of a tidal lagoon and the Portland city skyline, no one used it because it was too large (150 meters on a side) and too open.
Since the lawn sloped slightly down toward the water, I constructed a smaller level area (about 30 meters on a side) directly in front of the two buildings with a beech and two birches announcing the terrace’s outer edge. Although that edge was only a half meter above the remaining lawn, people immediately recognized the area as separate and small enough and enclosed enough for lingering. They soon brought a picnic table and Adirondack chairs out for steady use.