The engineers who designed the SRBs would have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railroad line from the factory happens to run through a tunnel in the mountains, and the SRBs had to fit through that tunnel. The tunnel is slightly wider than the railroad track, and the railroad track, as you now know, is about as wide as two horses' behinds.
So, a major Space Shuttle design feature of what is arguably the world's most advanced transportation system was determined over two thousand years ago by the width of a horse's ass.
According to Wiki,
Each is 149.16 ft (45.46 m) long and 12.17 ft (3.71 m) in diameter.
or aprox 3 times the width of a horses arse, Roman or not...
I wonder how much of the rest of that is nonsense too?