@Arkandel That depends entirely on scale of the game, and how detailed you want those rooms to be. There is no correct scale, but you know all that.
Fallen World recently dropped from 40ish? gridsquares down to 20ish, representing the island of Manhattan. Considering the size and scale and all, I think 20ish is a pretty good number, both to represent a big city and also to house the player base.
Another way to look at it is something like... What are the archetypal "areas" of a city, through the lens of a MU? A park, rich neighborhood, poor neighborhood, hip neighborhood, middle-of-the-road neighborhood, bordering the "wilderness", college area, government buildings. I might be missing a few, but that sounds about right. You then take that list and say you need 2-3 of most of those, to offer an option to players. That puts us nearish 20. So... It's a good number for a base grid!