It's 3.7 inches (3pi centimeters) in diameter weighs 2.9 ounces without the batteries, so it's fairly compact.
One of my favorite features of the 3pi is that the motors are driven off of a regulated 9.25 V, so your robot's performance doesn't change as the batteries discharge. This means that you avoid the common problem of finely tuning your robot with fully charged batteries only to see its performance change drastically when the batteries are low. The performance consistency also means you can even do limited dead reckoning by using simple timing. This is how my maze solver performed its 90 and 180° turns (I figured out how long to drive the motors at a given speed to achieve the desired turn, and no matter what the battery charge the turn is always the same), and it is also how I was able to drive faster on long maze segments while still slowing down before reaching the next turn.
- Ben
Oh, and here's a fun shot of one following a line in the dark:
