One way to look at the size of an organisation, is to value the size of the business based on its market capitalisation (shareholder value) which is the number of shares in the company multiplied by the share price, this gives the overall market valuation.

Another way to size the enterprise, is to base the valuation on the size of the balance sheet. Based on this methodology, here are the top 10 banks in the world based assets at the end of 2012:

1. Industrial & Commercial Bank of China (ICBC) £1853 billion. [China]
2. HSBC Holdings £1774 billion [United Kingdom]
3. Deutsche Bank £1756 billion [Germany]
4. Crédit Agricole Group £1753 billion [France] 
5. Mitsubishi UFJ Financial Group Japan £1710 billion [Japan]
6. BNP Paribas £1665 billion [France]
7. Credit Agricole £1609 billion [France]
8. Barclays PLC £1582 billion [United Kingdom]
9. JPMorgan Chase £1555 billion [USA] 
10. Japan Post Bank £1510 billion [Japan]

When you added these numbers together:
∑(£1,853bn + £1,774bn + £1,756bn + £1,753bn + £1,710 + £1,665bn + £1,609bn + £1,582 + £1,555bn + £1,510bn)

= £16,767 billion = £16.7 Trillion.

The top 10 banks in the world hold £16.7 Trillion worth of assets.