It’s close, but the BeanDuino may well be the smallest dev board we have featured on Tindie so far. This board is in fact a clone of another dev board called the digispark which is also quite tiny but it uses the PCB as a USB connector which adds a fair amount of area to the board. The BeanDuino has taken a different approach to adding USB connectivity by using a micro USB port on the board, saving on space.
This tiny board is Arduino compatible and can communicate via USB. You will need to flash the chip with the micronucleus tiny85 bootloader first to enable the chip to act as a USB device. There are 5 I/O pins on the board. Two of these go to USB communication only if the device actively communicates via USB. Of those 5 I/O you have 4 ADC pins and 3 PWM enabled pins. There is also an internal temperature sensor and an LED attached to pin PB1. Having a board this size communicate via USB coupled with a few I/O pins definitely has some cool project potential.