We shared the smallest board called PICO: the smallest Arduino board (0.6x0.6") a few weeks ago. Now, it is not true.

The developers of Atom X1 made smaller board, but with SAMD21 microcontroller. The size of board is 14.9 x 14.9 mm. It is really small development board. SAMD21 contains ARM 32bit Cortex M0+.
The campaign starts on indiegogo.

The board contains SAMD21, microUSB connector and also 2x5 pin hedaer.

The price starts from ?12.