You're trying to re-install economic partitions at the national level which currently aren't there; "that's how we do things" isn't an argument. "How we organize ourselves politically" with respect to economics has been as an integrated global civilization since the mid to late 20th century.
If economic partitioning is an unqualified good, why not go much smaller than country scale? If it's not an unqualified good, what makes national borders optimal? Doesn't it seem a little bit suspicious that a country grown over hundreds of years of political wrangling just happens to the mathematically optimal economy size?