I have a Schwinn 24" Uni and am thinking of purchasing a 36". I see some can order different crank lengths. How do I decide which length is best for my needs?
By telling us the needs.
say I want to go fast?
Or how about up a steep hill?
150s are good for beginners (to the 36) and for climbing steep hills.
125s are good all around. With a little experience, they’re fine for climbing, and are reasonably fast.
114s still useful on mild hills, and pretty darn fast
102s some people like them, but I don’t feel the speed gained is worth the loss of control.
When I first got a 36 I was a fairly strong rider and I found that 125s were a good fit for me. The learning curve is definitely steeper than with 150s. I eventually switched to 110s for everything, from mountain climbing to speed runs on the flats.
Everyone’s body is a bit different though, and the best way to know is to experiment. I recommend 125s for most people though, mike above me has some good comments on the different lengths as well 
I would go with what mscalisi said. We were actually very similar in speed during training for Ride The Lobster. There have been about a zillion threads on this topic, so there’s not much new to say, but plenty to read. Basically, speed is easy (short) steep is easy (long). Mixing them up, like riding in the real world, is more complicated.
I don’t ride 36" ungeared anymore, but when I did, my favorites were 114s. This was for riding on my mostly-flat local bike paths. Toledo is mostly flat also, so it’s probably a good size to work toward there. But I wouldn’t start out at that size.
My advice would be to go for 150mm cranks as they will be a lot easier for someone new to the 36" wheel. On the unicycle.co.uk 29-ers and 36-ers meant for people new to the size came with 150’s for that reason.
Experienced riders often switch to 125’s as they improve, but, many don’t as well.
Of necessity, a shorter crank always equates to less control, and, many riders who do go shorter, will often feel the need to install brakes, as, short cranks mean less control, especially when going down steeper hills.
Finally, a very common issue for riders new to the 36-er, is that of free-mounting- many riders, even when they’ve been able to ride the 36-er for months, find free mounting problematic, especially when tired after riding a while- 150’s are definitly better than shorter cranks when it comes to free-mounting.
An good alternative, is to get a 36-er with the option of dial hole cranks, meaning you can easily switch from 150 to 125 with the turn of a pedal wrench (and altering seat height).
Totally agree with Onewheeldave. Go for longer cranks for learning the 36er.
I bough a 36" wheel from the trading post section.
Now I am waiting for the KH frame and seatpost to arrive from Municycle.de
I am going to instal my 165/137 duall hole cranks, and I am definitely going to start with the 165 holes. The longer the cranks, the easier it will be to learn to tame the beast.
I want to ride the 36er off-road, so I will keep the crank lenght a bit longer than some mentioned earlier. Don’t think I will ever go shorter than 137 for off road.
Will see how things turn out…
125/150
I found 165’s a little choppy… the 150/125 double holes are great
For my 29er 165’s are a bit slow, I like 145’s.
For my new 36er, I think 165 is great for learning, and for off road.
On the road, I agree, 165’s suck.