For a largish Roman city of the early Imperial period, you should imagine a "civic package" of buildings: a theater, an arena, a forum-basilica with temple, aqueducts, sewers, and a stadium. Not every city had every building: for example, in Britain it was very rare to have a temple connected to the forum, Carthage didn't have aqueducts until relatively late (but did have massive cisterns), in Syria everything can be kind of weird, arenas are relatively rare in Greece, etc. Starting in the third century or so city walls and elaborate gates become more common, which used to be connected to the turmoil of the day but an equally good explanation is that walls were traditionally a privilege granted to municipia and so were an important status symbol--likely either factor played a role depending on the specific situation of the city.

Small towns are rather more complicated because it is rather common to define a "small town" as essentially a settlement without the civic package, and frequently without planned out grid streets. Some archaeologists use complicated and rather arbitrary numbers games to make the distinction eg, (a city is a settlement with more than x inhabitants), some prefer to use administrative distinctions (city's have major administrative functions) and some simply wing it and don't worry about coming up with rigorous distinctions. It is not terribly uncommon to find a small town without much in the way of civic structures at all, on the other hand, there are small towns in North Africa with extremely sophisticated and elaborate hydrological structures. So, again, it depends.

A hamlet, more or less by definition, its basically a small collection of domestic and agricultural structures with no civic buildings. They can be extremely complicated--are we looking at a hamlet? Agricultural sheds? A single house with dispersed structures? Would one family or multiple families live here? We can't really know. Trying to recover the Roman peasant is increasingly popular in Roman archaeology, however, so perhaps in a decade or so there will be a better answer.