The crank length is not necessarily determined by the length of the cyclist's arms. Most competitive racers in our Invacare® Top End® Force™ Series Handcycles are using shorter (170mm - 185mm) and narrower (14" - 16") crank arms and spinning rather than muscling through the gears.
The crank height and back placement are positioned to make sure the cranks don't hit the legs or chest rather than the crank length being compromised for this situation. A strong cyclist may want to have longer cranks to increase the leverage he/she is able to generate into the crank, but the longer crank arms could wear down a newer or weaker cyclist. Making smaller crank circles does not cause the cyclist to exert as much energy.
The crank width is more determined by the chest and leg widths. This is especially true with the Invacare® Top End® Force™ G Handcycle when the cycler's torso is used to crank and is sometimes positioned slightly over the crank assembly, and the S cranks are used to clear the chest and legs.
Generally speaking, it is best for a new handcycle owner to choose the standard lengths and widths listed on the order form. The crank arms are easily replaceable and, after the cycler has used them for some time, he/she will be able to make a better decision about these dimensions.