Are there advantages to a 26 inch MUni wheel over a 24 inch? If so, what are they? One other question, what crank size is best for downhill and a little hill climbing?
Consider the 26" more of a trails unicycle than an overall muni.
It rolls over bumps better.
a 26" will roll a bit faster and easier over rough terrain than a 24", but they are also heavier, and make hills take a little bit more effort at the cranks for the same result on a climb or descent.
Longer cranks give you more leverage over the rotation of the wheel, meaning that longer cranks make hill climbs or descents easier. Best crank size depends on your build, some tall people can spin big long cranks smoothly with no problem, I’m 5’7" so I wouldn’t go bigger than about 165s on a 24", but like I said, it depends on the person…and also what kind of hills.