I learned a fantastic engineering joke while I was in school.
The Space Shuttle's size and mass was partially constrained by how large they could make the big orange strap-on fuel tank, and that thing was constrained by how large they could make the solid fuel boosters which attached to it at the sides.
The boosters were built by United Launch Alliance, in Utah. Due to the routing of transporting the completed boosters, they had to be transported by rail from Utah to Cape Canaveral in Florida. No matter how they routed it, the boosters would inevitably have to go through a railroad tunnel. This meant the boosters could be no larger than the North American railroad gauge (width of track) for tunnels.
The North American railroad gauge standard is specified by the International Rail Gauge Standard, which is derived from a much older British standard, stemming all the way back to one of the very first railroads, the Liverpool and Manchester Railway (L&MR).
The railroad gauge there was originally designed as an improvement on old horse and cart road. They just laid rails on top of ruts, grooves that had been worn in between the two cities on a main road that had been there ever since those cities were Roman colonies. Horse and cart that traveled those roads initially were only as wide as they needed to be to accommodate two pulling horses.
So the size of the space shuttle is derived from the width of a horse's ass.