I think an economy is a region with a 'scope' that depends on the discussion at hand. You could speak about the ecnomy of Colombia in isolation, or you could look at the economy of Latin America as a whole. When comparing you'd compare Latin America as a whole with for instance North America, Europe or Asia.
It is common to compare the US economy vs the EU economy or all of Asia, not to compare the economy of Germany with Texas.
Usually regions that can be lumped together share a border.
To make the comparision more fair I think if you wish to compare 'apples with apples' as much as possible probably the inclusion of the NAFTA region would be more appropriate when comparing with the EU.
Technically US states are sovereign too :) ...and really, if you looked at the US states as separate economies you'd see many glaring dichotomies that would make you wonder how it can be considered a single economic entity.
No, Europe's not a country, but it behaves like one for trade and currency purposes and has no internal tariffs. Nor can Germany (for example) impose a tariff separately from the EU. So in this context, the EU might as well be a country.
Technically the USA isn't a country but an agglomeration (federation) of states.
Originally the EU was fully called the European Economic Community, the goal was to form a single economy. Whether or not it is a single country is not an issue here, the OP spoke about 'economy', not about 'country'.
Does it absolutely have to be a country to be relevant? The EU share the same currency. I don't know for sure, but I can't quite think of a reason why economies absolutely have to be countries.
