Yes, I almost expanded further on the healthcare point for that reason, but then realized the author seems as if they are possibly writing from the UK.
But I agree that "size" in this case definitely does not mean geographic size. Geographic size might be a factor for why America doesn't have better telecommunications or rail system, not health care.
I think most insurance providers are multi-state entities so higher costs in one state can be balanced by lower costs in others. But there may be some limits; if the ACA requirement that insurers use 80% of revenue on health care costs is applied per-state, they may not be able balance out a really unhealthy state. But this isn't an issue due to America's geographic or population size or even the quantity of states, it's that America has much more political power at the state level.
Just set up multiple systems. The US even has very convenient boundaries to split up the systems along: States, just like the EU.
Even most countries with universal healthcare further subdivides their systems. E.g. Norway (5 million people) operates regional health trusts that are largely independent. The UK (60 million) does the same on a larger scale (more trusts) and leverages the number of trusts to improve management of individual trusts by occasionally having successful trust take over the running of trusts where management is failing.
The "size" argument is almost always a total red herring.
Belgium is split right down the middle by language and culture, with many people supporting a split into two independent countries. Spain has Catalonia and the Basque country, two culturally and linguistically distinct regions that have long fought for independence. Ireland has spent most of the last century in a slow-burning civil war. Half of Germany was part of the Soviet Union until 1989.
When people say that universal healthcare can't work in the US because of "diversity", what exactly do they mean by that word?