Pure democracy could work in any size nation with the help of computers but in the real world, no computer network is secure enough to handle something so large and critical. If everyone in China or the US had a home comp and went home every night and voted on a bevy of issues, the results could be tallied in a day. Like I said though, hackerz would throw a big monkey wrench in there. Without comps, democracy in anything resembling a pure form, would simply require too much time to be effective in a nation of much more then a few million imo.
For the real world, I like parlimentary systems the best. The 2 party system in the US is a travesty, again, imo.