Is there a minimal tank/cage size requirement for reptile?

Well, I don't use the actual 2/3 rule, because total floor space is what matters, I am not sure if I said it so it made sense, I think the best rule of thumb for minimum cage size is:

length of cage + width of cage = length of snake.

This is a general rule, but, if you have a 6' jungle carpet python, and you put it in a 4' x 2' cage, you have 8 square feet, which is fine. With an adult, to an extent, bigger never hurts. And you also have to take height into account with some species. The above mentioned jungle carpet python, if that cage were 2' or 3' high, would be even better, as they are semi arboreal. It can be different with neonates. To big of an enclosure can actually stress them. You will see almost all snake breeders keep the babies in small enclosures or tubs for at least a few months, so they don't have to worry as much about getting them to start feeding.