The Guardian has an article which estimates LIBOR-based swaps to be $250 trillion – which is a lot of illions.  You can find the article here.  Writer Alix Bailin does a nice job describing the LIBOR setting process, which is remarkably like the Keynesian beauty contest.  Here you go:

The process for ascertaining Libor is really quite remarkable. It is not done by averaging the rates at which banks actually lend to one another for any given period of time. It is calculated by asking member banks (there are between eight and 20 for each currency) what rate they think they could borrow funds on the interbank market.

By the way, Bailin is a barrister at Matrix Chambers and used to trade swaps in the City.