Tested over millennia of drunken revelry to be just right.

keep to the Fen Causeway

There is an urban legend that Julius Caesar specified a legal width for chariots at the width of standard gauge, causing road ruts at that width, so all later wagons had to have the same width or else risk having one set of wheels suddenly fall into one deep rut but not the other.

In fact, the origins of the standard gauge considerably predate the Roman Empire, and may even predate the invention of the wheel. The width of prehistoric vehicles was determined by number of interacting factors which gave rise to a fairly standard vehicle width of a little under 2 metres (6.6 ft) These factors have changed little over the millenia, and are still reflected in today's motor vehicles. Road rutting was common in early roads, even with stone pavements. The initial impetus for the ruts probably came from the grooves made by sleds and slide cars dragged over the surfaces of ancient trackways. Since early carts had no steering and no brakes, negotiating hills and curves was dangerous, and cutting ruts into the stone helped them negotiate the hazardous parts of the roads.

Neolithic wheeled carts found in Europe had gauges varying from 130 to 175 centimetres (4 ft 3 in to 5 ft 9 in). By the Bronze age, wheel gauges appeared to have stabilized between 140 to 145 centimetres (4 ft 7 in to 4 ft 9 in) which was attributed to a tradition in ancient technology which was perpetuated throughout European history. The ancient Assyrians, Babylonians, Persians and Greeks constructed roads with artificial wheelruts cut in rock spaced the wheelspan of an ordinary carriage. Such ancient stone railways connected major cities with sacred sites, such as Athens to Eleusis, Sparta to Ayklia, or Elis to Olympia. The gauge of these stone grooves was 138 to 144 centimetres (4 ft 6 in to 4 ft 9 in). The largest number of preserved stone trackways, over 150, are found on Malta.

Some of these ancient stone rutways were very ambitious. Around 600 BC the citizens of ancient Corinth constructed the Diolkos, which some consider the world's first railway, a granite road with grooved tracks along which large wooden flatbed cars carrying ships and their cargo were pulled by slaves or draft animals. The space between the grooved tracks in the granite was a consistent 1.5 metres (4 ft 11 in).

The Roman Empire actually made less use of stone trackways than the prior Greek civilization because the Roman roads were much better than those of previous civilizations. However, there is evidence that the Romans used a more or less consistent wheel gauge adopted from the Greeks throughout Europe, and brought it to England with the Roman conquest of Britain in AD 43. After the Roman departure from Britain, this more-or-less standard gauge continued in use, so the wheel gauge of animal drawn vehicles in 19th century Britain was 1.4 to 1.5 metres (4 ft 7 in to 4 ft 10 in). In 1845 George Stephenson copied the gauge of British coal wagons in his area (about 1.42 metres (4 ft 8 in) for his new locomotive, and for technical reasons widened it slightly to achieve the modern railway standard gauge of 1.435 metres (4 ft 8.5 in).

And how is 568ml just right and 500ml too small? The difference is just over 10%.

I think the difficulty is psychological in how many people (in UK I grant you- but I've heard similar complaints from germans) approach drinking. Generally when you get a beer, you have a good drink of it to start and then slowly drink off the rest of it and then when it's nearly finished you polish off the final 2 cm in one go.

The loss of that 10% interferes with the middle "grazing" period of drinking. You have the first glug, then a couple of sips later, you finish it off. Gone in seemingly no time.

keep to the Fen Causeway