As wit h Arduino, it is becoming hard to keep up with Raspberry Pi variations. There area many sizes, clones and performance choices. And this is great.
Friendly ARM have developed pretty neat Raspberry Pi native board called NanoPi. As its name states, the size of board is 40mm x 40mm.
Dont be fooled by its small dimensions. It packs everything you may need to run small board computer:
CPU: Allwinner H3, Quad-core Cortex-A7 Up to 1.2GHz
DDR3 RAM: 256MB/512MB
Connectivity: 10/100M Ethernet
USB Host: Type-A x 1, 2.54 mm pin x 2
MicroSD Slot x 1
MicroUSB: for data transmission and power input
Debug Serial Port: 4Pin, 2.54 mm pitch pin header
GPIO: 2.54mm pitch 36pin. It includes UART, SPI, I2C, IO etc
PCB Dimension: 40 x 40 mm
Power Supply: DC 5V/2A
OS/Software: u-boot and UbuntuCore
More details are available at their wiki.
