Another interesting small mcu is the nrf51822 which is 3.5x3.8mm , it includes bluetooth low energy transceiver and dc-dc converter + 256K/16K flash/ram and it supports the mbed/arduino. It would be also possible to write/adapt a low energy mesh protocol for it.

Another interesting one: stm32f401, cortex-m4 ,84mhz,512KB flash, 96 KB ram, low power and can run python(micro python) - at only 3X3 mm.