I'd have to agree with offy. There would be too many issues with building a robot this small with ou any prior experience.
Here are just a few of them:
-Power: how will you supply enough power affordably without taking too much space/weight?
-Drive Train: how will you create an efficient drive train that provides enough torque for your robot, yet small enough with a low failure rate?
-Controller: how do you plan on creating a super small controller with minimal errors without many soldering skills?
Don't forget about your budget!
This is not to discourage you, but to understand the difficulties of creating such a small robot and also for you to think of what you really need for this robot. Infact, I think you should take on this project!
But first, do something simpler like the $50 robot, as offy suggested.
I hope you enjoy robotics!