In one, Timgad, they were only 70 to 80 ft.square.Often they measured 75 to 80 yds.

square, rather more than an acre, as at Florence, Turin, Pavia, Piacenza.[61] Occasionally they were larger, but they seldom exceeded three acres, and their average fell below the prevalent practice of modern chess-board planning.
[61] For Florence and Turin see below; for Piacenza, the plans on the scale of 1:1000 and 1:5000 in L.Buroni's _Acque potabili di Piacenza_ (1895).
In most towns, though not in all, the dimensions of the 'insulae' show a common element.

In length or in breadth or in both, they usually approximate to 120 ft.

or some multiple of that.

The figure is significant.