There is a general consensus that the standard size of an international postcard is A6 (148 x 105mm or 5.8 x 4.1 inches). The Universal Postal Union suggests a minimum size of 140 x 90 mm (5.51 x 3.54 inches) and maximum size of 235 mm x 120 mm (9.25 x 4.72 inches).
Like that of the oblong prison cell, its dimensions may vary but the space of the postcard is rarely a tabula rasa. The blank side is usually already populated with immovable objects, lines, boxes, instructions.
In keeping with the parameters of the postcard, the posts in this series will be intentionally short.