The only way to get a small circuit is to research exactly what you want it to do, design it in a electronics circuit cad program then send the files off to a company who will make the circuit board for you. They will even assemble the circuit if you dont think you can solder finer pitch components.
This means spending money, so you have to think whether the outcome is worth it.
As well as the circuit you have to look at actuators and sensors thet you would like remembering that usually the smaller you want something the more its going to cost.
I dont think that there is even a slim chance of using an mp3 or mp4 to use as a robots main circuit
