I found very interesting to read (from CME site) published information by BIS about the size of the derivative market.
The Bank for International Settlements compiles and publishes information about the total over-the-counter and exchange traded derivatives positions.
Even if there is a big fallacy to estimate the market by taking into account notional ammount, the numbers are still fastinating and incredible.
Interesting enough, derivative contracts remain concentrated in interest rate products, around 81% of total derivative notional amounts.
FX contracts are on the second place with 10.8% of total amounts.

Here we see Outstanding Notional Value of Derivatives Markets(in billions USD).

Here is Gross Market Value of OTC Derivatives (in billions USD).