I am unfamiliar with US messurments as we in the UK use pints and litres to messure liquid. And I know that US gallons are smaller then imperial (UK) gallons so I dont know how the US pints thing works!
But 12 fluid oz equates to 0.3549 of a liter. Are you saying that that would be the right size portion to serve. What size ladle would you use or do you use in your kitchen?
Do you think the portions should be differant if sold on its own in a cafe or sold as a starter in a 3/5 couse meal in a restaurant?
3/4 of a pint is 12 ounces. I would consider that a bowl of soup. 8 oz would be a cup of soup. You can put them in whatever container you choose. You can use what ever ladle size you want to make the portions. Or you can simply tip the soup pot and pour the soup in to the serving container. You don't have to follow any of the advice in this thread and serve any amount of soup you like and call it whatever you want.