Good lord, what do they teach you at college? 1 inch = 25.4mm. 25 is near enough.
So 5 inches = 125 mm
6 inches = 150 mm
Cranks of 150 mm or more are generally considered “long”.
Cranks of 110 mm or less are generally considered “short”.
Standard sizes are 80, 90, 102, 110, 114, 125, 140, 150, 165, 170 mm. There may be others (I have some 89s and some 127s somewhere).
What is a good size for riding a 24 fast on the flat? I have 114s on my 28 and ride it off road including hills. I have done so on 102s. 125s should be easy on a 24 with only a minimum of practice. 114s or 110s should make it go like poo of a spade. I used to road ride my old 24 on 102s.
But hey, Aspenmike - who is a better man than I - swears by long cranks and rides his Coker on 170s, whereas I use 150s. The moral: there is no right answer.
The mechanics are very easy to understand, though.
Long cranks give you more leverage. This means it is easier to control the unicycle on a down hill, and easier to power up a steep hill. In my experience, the downhill control is the most noticeable bit.
However, short cranks allow you to pedal faster, because your feet only have to go round in a small circle instead of a big one. You get more speed, at the loss of some control.
What is the correct balance between control and speed? One school of thought is that it is a bad idea to use a shorter crank than you can comfortably idle.
Another school of thought is that if you can learn to pedal fast with long cranks, you get the best of both worlds - the ability to go fast, but with as much torque and control as you need at slow speed or on hills.
A simple answer: try some 125mm (5 inch) cranks or, if you are feeling bold, some 114s.