There's always been consumer confusion about size which, with today's proliferation of different electrical devices, really does matter.
The cylindrical AA is still the most widely bought size, accounting for about 58% of value sales; the smaller AAA, which accounts for about 16% of value sales, sometimes causes confusion.
The rectangular 9V (bought most often for smoke alarms) and the bigger C and D batteries are easier to identify, but it's always a good idea for store staff to be fully briefed to help customers pick their way through the bubble packs.
The best way to think about the different product types is in terms of a hierarchy, with high-performance, high-cost at the top and low-drain, budget types at the bottom.
On the lowest rung are zinc-carbon and zinc-chloride batteries, for appliances designed for intermittent use. Many retailers think these products' days are numbered. The new flash and digital cameras have created their own demand for specialist high-performance batteries.