Polystyrene would be good, but youd need an awful lot of it plus theres disposal afterwards.
I’d say that about 5cm = 1 building floor would be good, where 1floor==4m (IE, 80:1). Then, a 2m building would have about 40 floors (160m) - not too bad. ‘Godzilla’ would then be about 35 floors tall…
This would also mean you could make cars using kitchen matchboxes - great for stamping on!
As a comparison, the Empire State is 380m (including radio mast) so that would be just over 4m in this scale - possible in a church hall, for example. The Chrysler building at 320m would be exactly 4m in our scale, and this is a whisker shorter than Auckland’s very own Sky Tower.
If you had a place with high enough ceilings, you could put radio masts etc on top of buildings to get a good city look.
Alternatively, use a 2.5cm/1floor scale to get 60-floor-high monsters and use normal matchboxes as cars (and have 80+ floor skyscrapers). Seems a bit tall, though, as I’d be able to look over the top of the worlds tallest building at that scale.