There are plenty of mid-sized cities with decent bus service and bikeability, where living car-free may not be the easiest option but is totally possible and preferred by many people. Austin, Seattle, Portland OR, Boston, DC...
There are far, far more cities below 200k and in those its very hard from what I understand. Even some large cities with 500k people have really bad public transport and bad bike infrastructure.
And to live a car free live, you need to live reasonably close to the center where housing is very expensive. From what I understand, the places that are best to live for walk-ability are usually the most expensive.