What is the maximum anonimity set of a CoinJoin transaction?
Regardless of the maximum size of a single transaction, a CJ anonymity set can be made arbitrarily large (bounded, of course, by participants) by building a multi-stage switching network out of joins.

This was described in the original coinjoin post:


  In particular, if you can build transactions with m participants per transaction you can create a sequence of m*3 transactions which form a three-stage switching network that permits any of m^2 final outputs to have come from any of m^2 original inputs (e.g. using three stages of 32 transactions with 32 inputs each 1024 users can be joined with a total of 96 transactions).  This allows the anonymity set to be any size, limited only by participation.


There is no atomiticy required between these transactions-- other than if some users drop out the anonymity set will be less than expected.