The height of the frame, the thickness of the saddle, and the length of the cranks define the "minimum leg length" required to ride a unicycle (the B value in the image below). 
Height of the frame + thickness of the saddle + length of the cranks = minimum leg length
If we grow and our leg length changes, we must raise the seat post as necessary (generally a few centimeters) to continue using the unicycle at an optimal height. The size of the wheel does not generally affect the "minimum leg length," but determines the difficulty of riding and the reactivity of the unicycle, as well as the level of power needed to ride. A 24" wheel has a shorter total gear ratio and is therefore easier and requires less power than a 27.5" or a 29" wheel. In order to offer the right frame for each age taking into consideration the physical structure and riding style of the rider, we provide frames of different sizes.
Mad4One Muni Frames - Aluminum
The Mad4One frame sizes we offer are: Small (S), Medium (M), and Large (L). Below the minimum leg length required for these frame sizes: Small: with 140mm long cranks, riders must have a min. leg length of 70 cm. Medium: with 140mm long cranks, riders must have a min. leg length of 73.5 cm. Large: with 140mm long cranks, riders must have a min. leg length of 77.5 cm.
URC Muni Frame - Steel CrMo-4130
In the URC product line, we offer the following sizes: Extra Small (XS), Small (S), Medium (M), and Large (L). Extra Small: with 125mm long cranks, riders must have a min. leg length of 64 cm. Small: with 140mm long cranks, riders must have a min. leg length of 70 cm. Medium: with 140mm long cranks, riders must have a min. leg length of 74 cm. Large: with 140mm long cranks, riders must have a min. leg length of 75.5 cm.
With shorter cranks, the minimum leg length is reduced. With longer cranks, the minimum leg length is increased.