:I've seen 2 main standars: 50,000 and 20,000 inhabitants. But then, there are cities (or towns) with less inhabitants that can be more relevant (say, because of tourism) than other bigger ones. For instance, in Río Negro Province, El Bolsón (pop. 15,537) is far better known and relevant than, say Cinco Saltos (pop. 19,819). A rule of thumb could be everything with more than 50 thousands, and other well known places. 
 :BTW, should we bold the capitals? /