The USA does not have the largest economy in the world by far:
http://upload.wikimedia.org/wikipedia/commons/7/70/Nominal_G...
http://upload.wikimedia.org/wikipedia/commons/7/70/Nominal_G...
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.
Looking at: http://en.wikipedia.org/wiki/File:Supranational_European_Bod... European economic zone is a larger economic group. And the UN is even larger, but hey that's another story.
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'.
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.
When you sum the GDPs of Euro using countries, you get 12.23 trillion dollars, or slightly less than the USA's GDP (http://www03.wolframalpha.com/input/?i=eurozone+gdp+vs+USA+g...).
How the size of an economy correlates with its resillience remains an entrirely different question IMHO.