When determining how much dishwasher soap to use, ignore the size of the dispenser in your dishwasher and whatever you do, ignore the suggestions made by the detergent-makers, whose interest it is to get you to overuse soap.
You probably only need a fraction of what detergent-makers want you to use. If that sounds fishy to you, consider that modern dishwashers use far less water than their ancestors, they also require less soap to do the job.
Back in the late 70s, the average dishwasher cycle burned through 11 to 15 gallons. This decreased to an average six to 10 gallons per normal cycle in 2000. These days, dishwashers use as little as 2 gallons of water; they simply use it better.
Proper dishwasher detergent amount
So, the answer to the $64,000 question?
KitchenAid says to use anywhere from 2 teaspoons to 3 tablespoons, depending on how dirty your dishes are and how hard your water is:
Postscript (now, this is important)
But dishwasher detergent needs a little bit of grime to activate. And some dishwashers are equipped with a “turbidity sensor,” which monitors how clean the water in the dishwasher is and adjust accordingly, eliminating cycles and not fully heating the water if the dishwasher doesn’t sense that it needs to.
Scrape, don’t rinse and try using less soap. Get the hardness of your water tested, as that can also be a giant leap toward clean dishes.