The classic Arduino Nano is the smallest board to build your projects with.
The Arduino Nano is a small, complete, and breadboard-friendly board
based on the ATmega328 (Arduino Nano 3.x). It has more or less the same
functionality of the Arduino Duemilanove, but in a different package. It
lacks only a DC power jack, and works with a Mini-B USB cable instead
of a standard one.
32 KB of which 2 KB used by bootloader
Analog IN Pins
DC Current per I/O Pins
40 mA (I/O Pins)
Digital I/O Pins
22 (6 of which are PWM)
18 x 45 mm