How was the diameter of the Space Shuttle Solid Rocket Boosters (SRBs)determined?

The SRBs are made by Thiokol at a factory in Utah.
The engineer who designed the SRBs might have preferred to make them a bit fatter, but the SRBs had to be shipped by rail from the factory to the launch site.
The railroad from the factory runs through a tunnel.
The SRBs had to fit through that tunnel.
The tunnel is slightly wider than a railroad track.
The US Standard Railroad gauge (the distance between the rails) is 4 feet 8 1/2 inches.

That's an exceedingly odd number!

Why was that gauge used? Because, that's the way they build them in England, and English expatriates built many of the US Railroads.

Why did the English build them like that? Because the first railway lines were built by the same people who built the pre-railroad tramways, and that is the gauge they used.

Why did they use that gauge in England then? Because the people who built the tramways used the same jigs and tools they used for building wagons, which used that wheel spacing, Okay!

Why did their wagons use that odd wheel spacing? Because, if they tried to use any other spacing the wagon wheels would break on some of the old, long distance roads.

Why? Because that's the spacing of the old wheel ruts.

Who built those old rutted roads? The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The Roman roads have been used ever since.

And the ruts? The original ruts, which everyone else had to match for fear of destroying their wagons, were made by the wheels of Roman war chariots. Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.

Thus, we have the answer! The United States Standard railroad gauge of 4 feet 8 1/2 inches is derived from the original specification for an Imperial Roman army war chariot. Specifications and bureaucracies live forever.

So the next time you are handed a specification and wonder what horse's ass came up with it, you may be exactly right, because the Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war horses!

A major design feature of what is arguably the world's most advanced transportation system was originally determined by the width of two horse's asses!