The U.S. may have been lucky because initially it was strung out along a seaboard which provided good transport and communication for the time, and as the U.S. spread into the interior, massive improvements in communication and transportation came along just in time, so we could have the cohesion that up til then was very hard to achieve except in a small state.

Some of the Federalist papers argued the opposite of what Montesquieu's point -- that a surplus of talented and ambitious people would tend to keep each other in check.

Anyway, Singapore poses a different question -- not whether small or large countries are best suited to democracy, but whether Singapore's (undemocratic) system could be made to work in a big country with rich and poor sections, and other wide variations of interest. Maybe Singapore, due to its nature could be administered well by one great CEO, but we haven't seen that sort of thing work well on a continental scale except maybe for short periods of time (usually followed by a traumatic succession crisis).