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.

I regularly freeze litre (quart) containers of stew, chilli, etc. from the slow cooker. I have done bigger containers but that's not very convenient for use. I tend to have about 3 litres to cool in total, most of that to freeze.

To cool big containers down quickly here's what I do:

• When cooking is finished, take the pot out and leave it with good airflow for at least a few minutes (typically while I eat). You could put a thermometer in there (at the edge) and leave it longer, so long as you keep it out of the danger zone with a bit of margin.
• Put the entire pot in a sink full of cold water for a while (10-20 minutes). Stir occasionally and change the water when it gets warm, at least once. (This is while I clean the kitchen after a batch cooking session)
• Transfer to storage containers. If large give them a few minutes in cold water.
• ideally put in the fridge, even if you're planning to freeze. If freezing at this stage make sure they're well away from already frozen foods to avoid partially defrosting anything. Smaller containers can go straight in the freezer (but again not touching other foods). If you have ice packs already frozen you can use them as a barrier.

It takes a little while to do this but you don't have to actually do very much. With smaller containers you can be a bit more relaxed. The approach is based on passing through the danger zone quickly, and not putting large amounts of heat into the fridge or freezer. A water bath cools much more effectively than air.

The largest safe container is only constrained by your refrigerated storage space.

In and of itself, simply moving your soup from the cooking pot to smaller containers is not a sufficient cooling technique in most cases. Cooling is important, not only for the product that you are preparing to store, but, perhaps more importantly, for the products in your refrigerator that you risk warming up when you place a hot product nearby.

There are many techniques for cooling, which can be found using the search bar. Here is one I haven't seen: If you want to cool a large amount of soup quickly, you can freeze water in quart containers. Then, with the containers sealed, float them in a large bowl or pot of hot soup. Stir gently. The soup will soon be cool enough for storage. Once cooled, you could stick the entire pot in the refrigerator, or transfer to smaller containers.

Ball Mason Jars are great for storing just about anything. They come in a huge range of sizes https://www.freshpreserving.com/gallon/.