I am shopping for an electric bike that is more electric than bike. It is a bit heavy duty and is designed to carry things but it still has pedals. The pedals have a really low gear ratio and are designed to help get the bike moving so that it takes a load off of the battery but there is enough room in the rear to put a 5 spd. cluster so that I can still get a bit of a workout while riding.

I have noticed that there is a big difference in the width of these ebikes - mostly to accommodate larger batteries. The width of the crank ranges from 16cm to more than 20. I have googled about this but only found info on the length of the crank arms. What is the optimal (from the point of view of the human body) width of the crankshaft? As the width increase, at what point does it begin to be harmful for the human body?

The measurement you are looking for is commonly referred to as, the Q factor.
https://en.wikipedia.org/wiki/Q_factor_(bicycles)
https://www.sheldonbrown.com/gloss_q.html
The very wide Q factors on scooters makes the pedals all but pointless. They are really only added to skirt laws limiting MUP traffic to vehicles with pedals.