Many would argue that there has never been a participatory democracy. In ancient Greece they recognized that a functional democracy would lead to the majority laying claim to unequal wealth distribution. When the United States was founded they reached the same conclusion as to that fatal flaw of functional democracy. In ancient Greece they attempted to solve the problem by reducing wealth inequality to prevent class warfare. In the United States they decided the best solution would be to reduce the democracy to a non functional state.
To be clear, what i mean by democracy in a non functional state is that public policy is invariably in diametric opposition to the majority attitudes, values, opinions, beliefs, desires and welfare. These kinds of woes cannot exist in a functional democracy. I shouldn't have to mention this here but the reason public policy is in such opposition to public will is because the ability of the unpriveleged masses to affect that public policy is effectively nil. At best a grass roots movement will accomplish nothing but waste, at worst it will just incite the rulers to double down on draconian handicaps and humiliation to deprive those masses of what little spare time they had to devote to political activism in between frantic bouts of competitive struggle for survival.
As for "optimal size", the more dense a population becomes, the more homogenous it becomes. It actually becomes easier to operate a functional democracy with increasing population, not harder, especially in the digital age, approaching the diamond age. So the "optimal size" is, "the bigger the better". It's also worth noting that the ultimate ideal of democracy is anarchy, with nobody in charge, nobody granted authority to coerce anyone for any reason.
