Incidentally, an argument you hear in Cyprus (which has a similar urban planning and car-dependency issues as the US but an extremely smaller scale) is that Cyprus is too small to be designed in an urbanist way.
Now, full disclosure, I kinda agree with that in a very specific way: there's a strategy, when talking about public transport in Cyprus, to pretend like the problem is the specific mode of transport. When Cyprus got buses, people boycotted them because "the bus-stops have no shade, no wifi, and anyway trams would be better". Maybe sensible on the face of it, but then when a tram feasibility study found weak support for tram and the bus infrastructure including the bus stops began tangibly improving, the tune changed: "trams are too slow, we need a subway in the major cities". They keep moving to goal post further and further into unnecessary infrastructure (some going even as fast as saying that Cyprus needs high-speed rail, which is ludicrous) so that they justify keeping the place car-exclusive.
In that specific way, I do agree that a country can be too small for some modes of transport. I don't think Cyprus needs more than buses and a few tram lines. Anything else is excuses to avoid doing anything at all.
Now, the USA is clearly big enough for high-speed rail and it has many cities that can support a subway system.