I'm looking for a real small but decent Arduino, that can fit in a watch but power a servo and a gyroscope
Look at the ATtiny85 MCU. It's a small 8pin device, and pretty easy to get up and running using Arduino code. You could also use the ATtiny84 which has more pins and still very small.

Start here: https://code.google.com/p/arduino-tiny/

Also, there are plenty of tutorials on setting up and programming them. 

Regarding a custom board, your least expensive route would be to design your own. Otherwise you would need to outsource the design.