Its not going to be really big, probably about 1 or 2 feet tall with 5 to 8 inch wheels, depends on what I can find. All the weight needs to go on top so the batteries will be on an elevated platform. Most people seem to start out with an inverted pendulum (ball attached to stick, stick mounted to robot). If I make it too big, I can make it smaller.
I was getting second Accel and gyroscopes just for experimentation purposes or even a second robot. I will only be using one of each for this bot.
After I get it balancing, I am going to want to connect a remote control to be able to drive it around myself. After that, I may try to get it autonomous with IR or Ultrasonic sensors.
Edit: There doesn't seem to be an Eval Board available to purchase for the 401, but the 300 series has one and you are right, the chips are much cheaper straight from analog.com