Personally, I wouldn't trust PetSmart with a dead goldfish. Not saying all of them are uninformed when it comes to animals, but the majority of the employees I've met have had no real training in animal care. The ones that do have a good head on their shoulders when it comes to information are usually self educated because they are passionate about the animals before working there.

As far as food size, the two general rules of thumb I've heard the most are about as big around as your snake at its widest point, and 10-15% of your snake's body weight. Here's where having a scale would come into play.