I believe that the limit would be 'out of the house'. Not grocery stores.
Think about it. Movie theaters, McDs, etc. don't have a true 'small' size drink. Their small is the old medium and in some cases the old large! Their default offering when they ask if you want a drink is the medium, which at Burger King is the old large.
When you go out to eat, most of us go through one 20oz drink just waiting for our foods, then one or two more during the meal, and sometimes one for the road, cuz, well, its free! That 64+ oz of drink in 1 hour.
While I think that the companies this rule will affect should be policing themselves, and offering these smaller size options themselves, there are some consumers out there that just have no control or live in the 'its the better deal for my money so that's what I must choose' mentality (speaking about family members here), whether or not they really NEED that much. How many empty 32oz + drink containers fill the movie trash cans after the movie? yes some are shared, most are not.
Honestly, you could put this restriction on the size of desserts at restaurants and such and I would be fine with it. The best thing Olive Garden did was introduce the dolcini desserts! I love those things. Just the right size to satisfy the craving without all the calories.