The Raspberry Pi is already pretty tiny, but if you need to slim it down even more for a project, then Adafruit has the guide for you.
Adafruit shows you how to get rid of the bulky Ethernet port, replace the USB ports with smaller ones, and even trim down the GPIO pins. The end result is a super-slim little Raspberry Pi that is much more suitable for portable projects, or designs where you need to get it inside a tight area. You'll void your warranty doing it, but if you want to give it a try, you'll find the full guide over on Adafruit.
Diet Raspberry Pi [Adafruit]