The smallest parasite will be the EZ430-T2012. It's tiny and cheap. It will need to be programmed in C (I'm avoiding assembly).Given that most new RF modules (like TI Chipcons and Zigbees) don't require much from outside microcontrollers, I could probablyget away with using a small parasite.
The next step up is the EZ430-RF2500T. It is more expensive (it includes an RF module). It can be programmed in Forth (SwiftX).
Each has its own particular appeal. I want to get back into Forth programming (I can't do this on the T2012 -- not enough RAM),but the T2012 is cheap ($3 a board) and hence has a certain ubiquity. If I go with C, I can code both using Protothreads as my primary means of abstraction.
The advantage to using Forth on the RF2500T is that it will force me to understand the CC2500 RF module. I can't rely on a C library. I get the same advantage using C on the T2012 since it doesn't have enough space for a C library (like Simpliciti).
Eventually I'll roll my own board.