This is not an easy question, it depends what you want to do with your unicycle. All the unicycles are sold with standard length cranks but you can upgrade your unicycle with different size cranks. As a general rule, the shorter the crank the faster you can ride, the longer the crank the more control and power your can have. Cranks are measured from the center of the wheel to where the pedals are threaded in.16" Unicycles
Can only be fitted with 100mm cranks or smaller otherwise the pedals touch the ground when riding.
20" Beginner Unicycles
114mm to 125mm cranks are a good length for most people learning to ride and introductory tricks. The rule of thumb is " the longer the crank arm the more control you have over the wheel."
20" Freestyle Unicycles
75mm are for advanced freestyle, 89mm are still too short for beginners but are good for advanced skills such as pirouettes, they are also ridden by some hockey players to go fast. 100mm cranks give a smooth fast ride but make idling harder, 114mm cranks are a good length for most freestyle tricks and hockey.
20"/19" Trials Unicycles
True trials riders use 137mm to 140mm cranks to maintain better control over the wheel and better pedal to tire transitions. 125mm are more common for riders that want to mix in more freestyle tricks into their street riding.
24" Beginner Unicycles (including Basketball and Race)
125mm cranks are smooth, fast, and still give you plenty of control for light tricks. 114mm cranks are very smooth and should only be considered if you want to go fast. 140mm and 150mm cranks are great for general all around riding.
24", 26", 27.5" & 29" Road Unicycles
114mm cranks make a great long distance machine for speed on flat surfaces but it can be very hard to idle and free mount. 125mm cranks are the go to size for general all around riding. Though 150mm cranks are considered long for road riding, those concerned more about control over speed find that this crank size is perfect.
24", 26", 27.5" & 29" Mountain Unicycles
150mm cranks are great for standard Muni riding. Riders looking for more control and leverage for hill climbing and technical terrain, the 165mm or 170mm is the length to use. Please note: for the 24" unicycle the longest crank size recommended is the 165mm.
32 & 36" Unicycles
150mm are ideal for a beginner learning to ride. For the intermediate to advanced rider, 125mm is the general do it all size. For riders who like a challenge and want to go as fast as possible 75mm, 89mm, 102mm or 114mm cranks will give you maximum speed with almost no control.