What type of small diameter motor/generator is best for for a hand-crank cell phone charger
A cellphone battery contains about 10-12 Wh (sometimes more). 

If this is an emergency charger, and we're in a  hurry, we will only charge it partially, doing only the CC phase where the battery accepts the maximum amount of power, and skipping the CV charge at the end.

In the CC phase most phones would charge at 1C and thus input 5V at a maximum of 2A. Most likely it will be less, around 1-1.5A if the onboard buck charger is of good quality, the power supply can supply the current and the phone identifies it as able to do so...

Thus someone will need to crank the crank on your charger for at least one hour while generating a tiny amount of power like 5-10W.

Who would want to do that when you can use a solar panel instead and not waste an hour of your time and get sore fingers? Unless the idea is only to provide a small amount of charge to make an emergency call.

In this case I would suggest to get rid of the crank, and replace it with a pull cord. 

Or you could try something like the GravityLight...

Anyway, back to your generator.

The idea for a generator is to have high \$ d\phi / dt \$ ie fast variation of magnetic flux in your coil, because this is what determines output voltage and thus power. For the same size of coil and magnetics, the more \$ d\phi / dt \$, the better.

Thus your alternator will look like a pancake. Think about a claw pole alternator for example. A magnetic circuit creates a number of poles on the circumference. Individual magnets can also be used, not getting into details here, but the idea is that at the same rpm, a wider diameter makes the poles run faster in front of the stator coils, which generates more \$ d\phi / dt \$.

This is why this alternator looks like a pancake. 

If you want a thin cylinder, you're out of luck. Won't work unless you use insane RPM. And you're unlikely to find one on the market, because everyone else will opt for the optimum shape... either pancake or a rather stout, not thin, cylinder.