I’m upgrading the cranks on my 24" and want to make it more road worthy. The area in which I live has rolling hills and some flat.
I would consult the Unicyclopedia, it has alot of good info to help you differentiate between the advantages and disavantages to each crank length.
(Look under the hardware section)
General rule is:
smaller = faster
larger = better hill climbing, control, maneuverability
What crank size you should use really depends on what kind of riding you plan on doing.
125s or possibly shorter. Bearing in mind that the standard crank length for the KH muni is 150mm and it’s used for the most extreme hills you can afford to go a fair bit shorter. Most 36" riders use 125s or shorter even when crossing mountain ranges so on a 24" you’ll still be able to hit any road hill.