Grocery sizes. In fact, a lot of them already have it on the items. Typically because a lot of products are also sold in Canada so they put the ml measurement next to the gallons.
Yeah is fl.oz on consumer containers _really_ seen as a volume measurement, i.e. thought of in terms of conversion "How many of these to a gallon", "what does a 6-pack weigh" and so on?
Or are they more like t-shirt sizes "I know I want a 16oz can to drink and I know how big they are"?
When I go to buy liquor it seems to already be mostly in metric, sizes are 500ml, 750ml, and liter.