my question: why is the coulomb (and ampere) the size it is today, which an outrageous size that makes it impractical for direct use?
The coulomb has that value because in the mid 19th century electrical engineers needed practical units for submarine cables and telegraphy.
In 1861 a committee of the British Association for the Advancement of Science was appointed to propose a system of units that included electrical and mechanical units. The committee defined two coherent systems for scientists, called electromagnetic and electrostatic units, and added several "practical" units that were decimally compatible with the electromagnetic units, for the electrical engineers involved in telegraphy and submarine cables.
In 1861 the electrical engineer Latimer Clark suggested to the committee that practical voltages were in the range of 1 to 10^6 volt, resistances of conductors and insulators were in the range of 1 to 10^8 ohm, and the smallest current was about 10^-3 ampere (translated to modern units). He proposed names like volt and ohm for these practical units, and the prefix mega for 10^6. The unit of charge was unimportant for submarine cables and telegraphy, so nobody cared about its practical value, it merely had to be coherent with the unit of current.
In 1881 the International Metre Convention adopted the practical units and their names: volt, ohm, and ampere. These practical units are the ones that scientists and engineers are using today.