There's a new Raspberry Pi available, the Zero:
https://www.raspberrypi.org/blog/raspberry-pi-zero
It measures at 65x30x5mm, costs $5 and has identical pinout to Model A+/B+/2B. Full specs:
- A Broadcom BCM2835 application processor
- 1GHz ARM11 core (40% faster than Raspberry Pi 1)
- 512MB of LPDDR2 SDRAM
- A micro-SD card slot
- A mini-HDMI socket for 1080p60 video output
- Micro-USB sockets for data and power
- An unpopulated 40-pin GPIO header
- Identical pinout to Model A+/B+/2B
- An unpopulated composite video header
- Form factor, at 65mm x 30mm x 5mm