At least in the United States, manufacturers are supposed to abide by the FDA's "reference amounts customarily consumed per eating occasion" when determining serving sizes. There are some notable rules for "single-serving" containers, rounding, and ingredients.
The base reference amounts are listed on the FDA's website.
The exception for single-serving containers comes into play when a food/item can reasonably be consumed in a "single-eating occasion". This is why you sometimes see a 12oz can of soda labeled as a single serving, while a 20oz bottle is listed as 2.5 (8oz servings).
Foods which are generally used as ingredients instead of being consumed directly (flour, baking soda, salt, etc.) will have "serving sizes" in common measurement amounts, rather than common consumption amounts.
Manufacturers regularly undergo FDA inspections, and will be issued warnings/fines if the standard is not met. For instance, if you don't measure your blintzes right, you may get one of these (see section 3).
I don't know the situation in other countries.