Pet shops will sell anyone anything, from my experience. Thats why people need to get used to seeing the size thier snake eats, so you can identify the size (no matter what is written on the package) visually. If you are buying without seeing the size, best you measure the length (minus tail.... from nose to base of tail) and the weight. That way you can ask the supplier for 'rats that are 4cm from nose to butt.... approx 40g'

To me that is about a weener.... but at least if you give the dimentions, there is less chance of a mistake.