I suppose it could be likened to a planet sized (though not shaped) city. Something that eclipses all but the most massive and populous Hiveworlds. Even then the Dark City works on and through more dimensions than any Hiveworld in Realspace and it also has more room to expand and contract.
I agree that it would be best to view the Dark City as many hundreds of cities that has grown so large that they've merged together (like the City of London and the City of Westminster, etc), added to which we have other cities that although many thousands of lightyears away from each other in one sense, they're also just a step away from each other through the various webway portals.
It that respect I suppose the Dark Eldar's realm could be regarded as an empire of sorts, with territories scattered far and wide, but with tributes flowing back to the metropole, the economic / political centre. It may not be that Vect could be described as an emperor in the way that we most often use that phrase, but then empires don't always need emperors, oddly enough. Vect is the paramount power in the Dark Eldar's civilisation and I doubt many would be stupid enough to deny him something were he to ask for it. Then again, I doubt he would be stupid enough to ask for something that he knew would likely destabilise the system he set up by causing a massive and unnecessary war amongst the Kabals.
Even so, I think the Dark City would be a bit more politically unstable than most Empires (which is saying something). The Dark Eldar are by orders of magnitude more selfish, conniving and ruthless than any human, and I imagine that their political interactions would leave even the most cynical and imperialistic Machiavellian plotter utterly horrified.