The 16 inch unicyle is a beginner freestyle unicycle for children and small adults. I had to buy a much longer seatpost and a new saddle designed for bigger adults (I'm 1.85m tall). The cranks are very short which makes the unicycle harder to control, but there is less risk to touch the ground with the shoes (if you have bigger feet).

The 20 inch unicycle is a common unicycle with a narrow tyre (tire) and front saddle handle which makes hopping a lot easier.

The 24 inch unicycle is a cheap unicycle with long cranks which offer better control. This unicyle is designed for both freestyle and road use (but not perfect for any of them). The distance from the ground is higher. It's a beginner unicycle for taller persons.

I feel comfortable with any of them but I prefer the 20" unicycle for maneuverability (I think this is the best wheel size for adults to learn any unicycling skill) and the 24" unicycle for both stability and speed. For me it's hard to choose between 20" with shorther cranks and 24" with longer cranks (both of them have pros and cons, but it is not a big difference between them).

Also watch these videos:
"Old 18 Inch Unicycle":    • Old 18 Inch Unicycle  
"Unicycles and Handles":    • Unicycles and Handles