What size should i get

I am 13, but I am only 5 feet tall. I have long legs though. I want to learn because I’m sick of the bicycle, I want to ride something that is more fun to get exercise (I am a couch potato ATM) and makes me stand out. I want to learn to ride a unicycle with a big wheel so it goes faster, but not too big so I can’t get on it. Is 20 inches good? We found one on craigslist.

At your size, I would recommend learning on a 20-inch Uni. To go fast on a Uni, you really need a 26-inch or larger wheel, which is not conducive to learn on. So, buy a cheap 20-inch to learn with and then decide what type of riding you want to do most before you buy your second Uni.

What size?

** All of them!! **

Make sure that your inseam is at least as long as the “minimum inseam” of the uni you found on craigslist. (Seat post length and wheel size will determine a minimum inseam length to ride that particular uni.)
20" is what everybody in going to say to learn on especially considering your age. I can’t say I disagree. I learned on a 24" which provides a little bit more speed because of the larger wheel but then again I learned when I was 31 years old.
The fact is most unicyclist have more then one uni…but most of us started with just one. The 20" is a good idea, go for it. The real bonus is a 20" is ideal for doing tricks, spins, learning to ride backwards, wheel walking, and the like. The best trick riders and freestylers use a 20" uni.
I personally disagree with the idea of getting a cheap learning uni with plans to upgrade later. Buying quality will give you a better experience and “the quality will be remembered long after the price is forgotten.”

Good luck to ya buddy and welcome.

I would agree, at your age/height, 20" should be optimal. A 24" will probably be too big for you to efficiently learn on, but would be doable. Going as small as a 16" might work, but will be too small for you once you get moving, and grow more.

Get a cheap 20" in decent condition and then, once you learn, you will have a better understanding of what kind of riding you want to do. Then you can buy another wheel accordingly.