Big fish or little fish?

[David H]

When you buy fish from a pet store, for example a Corydoras catfish, is it wiser to buy a small fish or a big fish? In my mind, a smaller fish implies that it is younger and it will last longer, but a bigger fish might be heartier. I'm about to buy a pack of Corydoras Pandas from a local fish store and the tank there has them in all sorts of sizes and walks of life.