The smallest gear depends on the diameter of the shaft which limits the practical diameter of the gear. As far as the largest gear, there theoretically is no limit since you could have a very large gear with the appropriate shaft hole to fit the motor shaft. How practical a five-foot diameter gear on a CIM would be, I don't know.
To find a good selection of gears in the range you might be interested in, check the gear catalog from Stock Drive Products. That would give you an idea of what is out there in the size you might be looking for.
http://www.sdp-si.com/estore/CoverPg/Gears.htm
Dr. Bob
Chairman's Award is not about building the robot. Every team builds a robot.