Complex sentence: It was slightly smaller than a modern U.S. quarter with diameters 22 mm (1793), 23.5 mm (1794--1836) and 23 mm (1840--1857).
Split into simpler sentences: It was slightly smaller than a modern U.S. quarter. Diameters are: 22 mm (1793), 23.5 mm (1794--1836) and 23 mm (1840--1857).