As far as alpha 20 goes:
1. No, you don't. And there's some variation on the number of unique POIs per map, though the bigger the map the smaller the variation.
2. In principle, changing the number of cities has no bearing on the size of cities. However, if the map is too crowded then you might get less big cities (the ones with downtown), and the cities might become smaller as well. That goes not only for cities, but for other things like mountains, hills, lakes, rivers, cracks and craters. Increasing map size helps offset that.