Well, people seem to be talking about various crank lengths and I’m just curious as to what that’s all about? What are the pros/cons of having longer/shorter cranks?
Longer cranks give you a bit more torque, increasing stability. For steep uphills and downhills and trials riding, longer cranks are usually prefered. Shorter cranks are faster however. They are also capable of spinning much faster for street riding. Many distance riders and most street riders use shorter cranks.
Thats a pretty brief description, but I hope that helps a little.
Look at it as the crank being a leaver.
Here is a picture to show why longer cranks are better for trials.
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crank length controls pedal speed
The shorter the crank, the slower the pedal will move at any given MPH. That means your foot doesn’t have to kick as fast, to put power into the pedal, as it would have to if you were using a longer crank. Of course, mechanical advantage is reduced with the shorter lever.
It is wrong to think of crank length as gearing. We are stuck in one gear. So we fuss about crank length, trying to make the best of it.
Sadly, the 10 speed unicycle , or even a 3 speed, doesn’t exist.
1) So would that mean that a 175mm crank on a MUni is better then 150 or 125.
2) Wouldn’t that be a schlumpf unicycle??? 
You seem to be contradicting yourself. If long cranks make you go slow, then the mechanical advantage of speed is increased with short cranks- not reduced. It’s only really steep hills that short cranks lack leverage- and a lot of them can still be climbed if you maintain momentum.
schlumpfs only have one gear. Well i mean one non 1:1 gear ratio.
and 175 cranks on a muni would give you loads of control and be easier uphill and stuff, but you would be really slow.
Well dependent upon what sort of terrain you are riding. It’s mostly about personal preference.
1) Really, i didn’t know that considering you can adjust the speed and whatnot. Well there you go, i learn something new everyday, 
2) Yeah i guess, but i think 150s would be a good choice too also.