Get this: http://www.unicycle.com/shopping/shopexd.asp?id=1050
and ask for the $50 dollar upgrade to the KH moment cranks.
By “riding around” how far do you think you’d want to go? I mean if you aren’t worried about getting there fast then the 20" wheel is fine.
24"
if u have to travel long distances to get were u wanna ride u might want a 24"
it definetly helps when its hot and u wanna get there fast a 20 works great but is obviuosly slower as to what u can do on 24 is really up to u trials are harder because the wieght and the wheel base is larger but u can still do trials or street u dont have to have a 20"
Well a…
It all depends on what you’re most likely going to be using them for.
It’s pretty hard getting from 24" to 20". You’ll feel like you’re not going anywhere. The thing is just so damn slow
But wouldn’t the opposite work if you were going from a 20" - 24" 
Also you’re right about the 24" - 20". Its almost instantaneous on how sluggish it becomes.
I started with a 20" and then went to 24". It was awesome 
Then i got back to 20" and now I got a 36", so I can tell you the speed difference sucks:D
I once went from a 12" Sun which is what my little cousin has to my 40" coker and there was a massive speed difference. (as there would be)
I got used to the 36", didn’t ride my 20" for quite a while. But once i had to ride to the train station with my 20". I thought I was going to die:D
I know the feeling. 
I have decided that I would like a 20" uni. The one that I am looking at is the nimbus trials isis.
I was wondering if someone could tell me how long a seat post I should get. I am 5’9" and have a 31 inseam.
Get the longest available, and cut it to fit. Cut an inch at a time, so you dont over cut the post and end up not being able to raise it when you ever feel the need to raise it.