On bulk carb-y foods like breakfast cereal a serving is pretty consistently 2 oz by weight. For milk it's a fluid cup.

There was a push some years ago to make sellers define a "serving" on the nutrition label of an obviously single serving as one serving, without "1/3 donut" nonsense. For example larger bags of chips like you get in the gas station, or 20 oz cokes, that had been labeled "2.5 servings" and had nutrition info for 8 oz of Coke. A really egregious example was PAM spray which was advertised as zero calories for a 1/4 second spritz - because they round to the nearest five calories. If a serving is one slice of bread that's pretty easy to compare sourdough with WW and see that the WW has way more fiber but also way more sugar. This is one of the several things that caused the unsatisfying downsizing of candy bars