I tend to follow http://www222.pair.com/sjohn/blueroom/demog.htm so:
Village: 20-1000
Town: 1000-8000
City: 8000-12000
Big City: 12000-100000
But I also made a village generator for IPP and I wanted more words, so I also use Thorp for 20-80, Hamlet for 80-400, and then Village is 400-1000. Thorps can be seasonal, e.g. a logging camp or hunting or fishing camp which is only inhabited part of the year. A Hamlet could be seasonal too although it's a bigger deal. Also Commune which is a religiously oriented settlement which I pegged at 80-600. I took some guidance from Skirmisher Press's City Builder book for these.
In practice, a settlement will retain a name for a while after it grows or shrinks. So the name might reflect what it was 1-3 generations ago, rather than what it is now. Thus you might have a town on the decline with only 800 people left, or a bustling and growing town spilling over its walls with a population of 10,000. All the numbers should therefore be a bit fuzzy.
Metropolis and megalopolis have a very modern sound to them, I wouldn't use them. Unless gigantic cities are strangely common in your setting, I think it's fine to just call them cities. These will be places that everyone has heard of. If your giant city is called Leontunia, you can just say "there are cities and then there is Leontunia", and people should understand.