Colin,
Please check out:
http://www.eia.doe.gov/emeu/cabs/euro.html
The figures for the current EU can be found under "Economic and Demographic Indicators for EU Countries ".
The total GDP 9,225 trillion USD
If we add the new EU member states: 400 billion USD
Total: GDP 9,625 trillion USD
This is not including applicant member states.
(Sources: DRI-WEFA World Economic Outlook, World Bank)
I don't see the EU becoming the biggest economy in the next 10 years as alot of reforms are still needed, which takes time.