I am going to get my first unicycle soon and was wondering how fast a 20" unicycle is, or if I should just get a 24".
That depends on if you plan to mainly ride around your beet farm or in the city around your Office in Scranton, PA. 
Seriously, if you’re even asking about how “fast” is a 20" you’d be better off with a 24" if you want to ride any distance. However, to learn on a 20" is probably your better bet.
Don’t get a 20" if you want speed. 24" or larger is definitely the way to go. Even with 89mm cranks on my 20", someone could easily out run me. It’s not fast.
Haha. Thanks guys. But I plan to do some minor stunts like hop off ledges/stairs. Would that be okay to do on a 24"?
if you get a good enough 24" than you should be fine for jumping, etc. Some people even prefer a 24" for street style riding.
Well my inseam is only 31", is that enough to use a 24"?
I have a 31" inseam and use both a 20 and 24. If Ii only had one unicycle I would go with the 24.
How fast is a 20" unicycle?
In a word, not. 
If getting someplace is more important to you than jumping off things, definitely 24". Or bigger, though you can still do loads of jumping on 24".
Okay I was just wondering because I heard of this guy who like bent his rim flat from jumping and hopping and whatnot. So I’m probably going to get a 20" one to learn on then probably get a 24" later.
How many inches is the post out on the 24? I’m going to order mine soon!
It’s not that hard to learn on a 24 - there are benefits.
I learnt on a 24 and one of the things I felt was beneficial was that when I swapped to a 20 later on (for street and hockey), I felt a lot more stable on it and was less afraid to try out tricks. For me, in the learning phase, I didn’t have any expectations so after conquering the height and weight of a 24, switching to a 20 felt less intimidating. Also, learning on a 24 is likely to make you more versatile - it’s easier to adapt to a 20 than someone who learnt on a 20 to adapt to a 24.
Assuming you pedal at a rate of around 90 revolutions per minute (quite fast) you’ll be moving at 8 km/h (5 mph) on a 20", and at 10 km/h (6.2 mph) on a 24".
Juan Reyero
unarueda.com