Unicycles for beginners and children
Most Beginners start with a 20 inch Unicycle. It has turned out that this size, fits the best and is the most suitable.
The children who are too small for a 20 inch will need a 18, 16 or even a 12 inch. However, pick the largest Unicycle that fits, but one that is also small enough that they can comfortably reach the pedals.
Most Adults begin with a 20 inch Unicycle. If the saddle can not be raised high enough, there are longer Saddle posts available. However, Adults can begin as well with a 24 inch, if they already know that they want a 24 inch. The larger Unicycles are very seldom taken by beginners due to the extreme challenge.
There is no rule of thumb which Unicycle size, fits for which height. This is decided by the inseam of the child and minimum inseam of the Unicycle. The minimum Inseam on each Unicycle is different, even if the Wheel sizes are the same.
This is how to measure the Inseam:
You need to take a book (DVD case) and put it between the persons legs as if they were sitting on a Unicycle ( the same as sitting on a Bicycle seat).Now squeeze the Book between your legs, holding it as tight and as parallel to the ground as possible. Now with a measuring stick / measuring tape, measure from the ground to the upper edge of the book (please measure with shoes):
Unicycle dimensions for different intended purposes
Freestyle / Tricks
Here are mostly 20 inch ridden. Smaller ones, only if they can not fit on a 20 inch wheel (see explanation for the beginners on top).
Travel / Road
For touring, the policy is the bigger, the better. With the larger wheels, one rotation means the farther you are going. Therefore you can go faster and lay down more distance. The 29 and 36 inch wheels are the specialists. But also 24 and 26 inch are being used.
Hockey / basketball
Here are mostly 20 and 24 inch ridden, due to the agility.
Trials is overcoming anything that gets in your way (stairs, Park benches, Garbage cans, stumps, palettes ....) riding a 20 inch Unicycle with a Fat tire 2.5 - 2.7 . Also 24 inch Unicycles with a fat 3.0 inch tire can be used. Then, indeed, they are a little heavier and not as agile, however, they are a All-Arounder which can do more than one thing as for example small tours, Muni, Trials.
Street is a combination of Freestyle and Trial. Here 20 inch Unicycles are ridden, almost always with a fat tire (min.2.5 inches).
Cross is the riding though fields and riding on unpaved ways. Here bicycle dimensions of 20 - 29 inchare ridden. The bigger wheel is in the advantage here, it can be ridden like the touring Unicycle faster and further. However, if you want to remain in the usual size 20 or 24 inch "to each, his own ". You can also ride a 20 inch in the woods.
Muni and Freeride it is like riding with the cross, except here it may get a little down and dirty. There are also some jumps or obstacles, therefor Munis must also be more stable than a cross. Here 24 inches with a thick 2.6/3.0 tire are very often ridden. More and more rider goes for 26 inch Munis because there are so many differend tires from the bike market.
Many go from "normality" also with a 20 inch. This is a disadvantage for the rider. You are going to have trouble rolling over obstacles, jumps and are not going to have the speed.