Is it possible to change the lengths of bike cranks?
Yes, you can change the length of the crank arms. If you are going to replace crank set, you need to make sure it is compatible with your frame. There are a number of different interfaces between the cranks and the bottom bracket.

Some common sizes are (many cranks will have the size stamped on the back of one or both of the arms):


165 mm
170 mm
175 mm


The size is the distance from the center of where the crank attaches to the bottom bracket to the center of the pedal axle.

If that is enough, you can just swap your crank set for different one. If not there are companies selling specialized shorter cranks or crank shortening devices 

Lastly, kids bicycles have much shorter cranks, maybe it would be possible to use some of them on an adult bicycle.