They are different sizes than the normal AA, AAA, C, D, etc... that you are referring to.
List of battery sizes - Wikipedia, the free encyclopedia
For comparison:
C - 50mm long x 26.2mm in diameter
18650 (aka 168A) - This is 65mm long x 18mm in diameter (this measurement is where it gets its name from.
14500 - 49mm long x 14mm in diameter
10440 - 44mm long x 10mm in diameter
CR123A (RCR123A, 16340) - This is 34.5mm long x 17mm in diameter
CR2 (15270) - This is 27mm long x 15.6mm in diameter
Not sure if this is a hard and fast rule, but if you notice the battery number (ie 18650) they generally tend to break down into the dimensions you are requesting. While this may not be 100% accurate it should give you a general rule of thumb to work with...
First two digits = Diameter
Second two digits = Length
Add a zero
So without looking it up, we could assume based on the above information that the 18500 is going to be 18mm in diameter and 50mm long.