Smallest GSM module available

Hi, I want to use the Arduino Micro for a GSM project. I am looking for a compatible very small GSM/GPRS modem that will work and connect to the micro. Does anyone have any experience of which would work ?

Thanks

Most GSM modules would work with the Micro. What do you want to do with the module? Just sending SMS? Sending data? Which pins are free on the Micro in your project?