On the smaller cells, you need the digits.
First two are dia, the next 2 or 3 are length or thickness.
A 2032 (you've seen these in fobs and other things) is 20mm dia, and 3.2 mm thick.
So a 18650 is approx 18mm x 65mm (like many things, this is not exact)
A 21700 is correct but so is 2170 since there is a not a 21mm x 7.0mm cell yet.
They should probably stick to the 21700 designation. There is a 2354 which is pretty close to what a true 2170 would be.