Thank you for your reply. I am totally confused. Maytag only lists one drain cover being available. Part number 99001795 which they say is for Series 10. They don't list one for series 15.
You are correct, I have a Series 15 which according to the Maytag parts list takes part number 99002281 - the small (less than 1/4" diameter) ball.
Here is a picture of my drain cover (with the ruler) which seems to match the ones you can see from the parts dealers websites. It just doesn't look like it will work.
I also attached a picture from the parts dealer's website for comparison. Looks the same as mine to me.
I know other dishwashers use similar check balls. I was hoping someone might have worked on one or had some parts laying around and could steer me to a make and model which would work.
Do you know of a substitute which is close to the same size?