The main slogan is: ‘keep a square as small as possible’. On a plan a square always looks smaller than it will be experienced in reality. Alexander relates the size of the surface to the number of people on it. To make a square feel pleasant and not deserted, he alleges there is a maximum of 90 square meter a person.
There is a big difference between the maximum size of a square, confirming Alexander or Sitte. Alexander claims the squares width should be 14 til 18 meters, with a absolute maximum of 21 meters (the grey rectangles in the illustration aside). With this distance, you can still recognise faces. With the maximum proportion of 1:3 a square could be 42 til 54, or maximum 63 meters deep. Sitte presumes a square can be a lot larger, at maximum 58 meters width and 143 meters deep (the largest square in the illustration aside). This seems to be very large. With 90 m² per person this maximum square needs 92 pedestrians for preventing it to feel deserted.
A large square would fit in a big city, but in normal circumstance I recommend a square with a wide smaller than 30 meters (the red rectangle).
Sitte also describes the relationship between the squares depth and the height of the main building. “A good proportion between the size of a plaza and that of its buildings is of primary importance”. Roughly, the depth in front of the main building is 1 to 2 times the buildings height. Contrary, the main building should be minimum half the size the depth of the square in front of it.
– Out off the theories and analyses from this site I formed some notes with my own perspective on urban squares to help making choices at designing them.