Thanks and they our, Mini Metal Gear Motor Located here, http://www.solarbotics.com/products/gm18/
then your CAD is definitely NOT to scale
those motors are tiny, in fact I have one in front of me right now and its smaller than my thumb . Lets call the length from the shaft of the motor to the end of the motor letter X. The Axon's length is only 2 times as long as X.
Whats kind of funny is that I used almost exactly the same parts that you drew up in this robot
That robot drawing is perfectly to scale . Notice the size of the Axon in the middle. Notice the size of the motors and wheels. I used the exact same motor type as you plan to(though mine is a different gear ratio it is the exact same size). I also used a standard servo , just like you did. I also used a 6V battery pack. Only difference is that I put a camera , and you want a Sharp IR sensor.
Follow that Google Sketchup tutorial I sent you on how to make robots to scale. If you need a bit more help just post a question on this forum, and me or the other guys here will be happy to help you.