How to Intercept a WCF Message to track message size

As we are building out our mobile WCF endpoints we want to be able to track/trend the size of messages which are being sent to our phones.  We want to do this for multiple reasons 1) to trend our message sizes over time to make sure we are being good little developers 2) to be able to report and measure the amount of data we are sending to each of our mobile devices because bandwidth is not free.