We shared the smallest board called PICO: the smallest Arduino board (0.6×0.6″) 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 2×5 pin hedaer.
The price starts from ?12.
https://www.indiegogo.com/projects/exen-mini-smallest-32bit-arduino-comp-dev-board-diy/x/18055658#/