What is the biggest safe container for soup?
You said the containers are plastic so you have already determined the size of the container. If the containers are clean and sanitary then they are safe. The only important part is cooling the soup down fast enough so it stays out of the "danger zone"; that is, above 40F for more than two hours. So however long that takes will take experimentation or applied physics knowledge or experience.

You can gain experience by heating some water to a simmer as you would for soup (I think). Then see how long it takes you to cool it to 40F. If you can do that in less than two hours, you're cool and safe.