Would you be surprised to learn that the size of the Space Shuttle is determined by the size of an average war-horse during height of the Roman Empire?
This is because the size of the Space Shuttle is more-or-less determined by the size of the Solid Rocket Boosters which lift it from the launch pad. These were built in a factory in Utah. To get to Florida the boosters had to be shipped by train from the factory, the railroad from the factory runs through a tunnel in the mountains, and the boosters were therefore designed to fit through that tunnel. The tunnel is only slightly wider than the railroad track.
The standard railroad gauge (distance between the rails) in the United States is 4 ft 8.5 inches wide, because that's the way they built them in England, and the US railroads were built by English expatriates. The English railways were built by the same people who built the pre-railroad tramways, and that's the gauge used by the tramways, so that's the gauge they used to build the railways. Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.
Interestingly that wheel spacing was used because if they tried to use any other spacing wagon wheels would break on some of the older, long distance, roads. Because that's the spacing of the old wheel ruts.
These roads were built by Imperial Rome for the benefit of their legions and have been in use ever since. The original ruts, which everyone else had to match for fear of destroying their wagons, were first made by the wheels of Roman war chariots.
Thus, we have the answer to the original question. The United States standard railroad gauge of 4 ft 8.5 inches derives from the original specification for an Imperial Roman army war chariot, which was designed to be just wide enough to accommodate the back-ends of two war-horses.
Hence the size of the most advanced spacecraft ever built by mankind depended crucially on the size of the average rear-end of an Imperial Roman war-horse.
The moral of this story? Be careful when building standards, they last forever. It doesn't matter whether they are good and sensible standards, or bad and ill thought out. Standards never die.
Now, does anyone want to guess why the size of an Amazon Kindle was determined by the size of an average medieval sheep?