The “box” in Pandora’s Box is traditionally understood as a pithos, a large Greek storage jar. In Hesiod’s account, Pandora receives a vessel, but the original Greek word is not the modern word “box.”
A pithos was a substantial ceramic container used in the ancient Greek world for storing commodities such as grain, oil, or wine. These jars could be large enough to stand on the floor and were very different from a small wooden box. The vessel’s practical storage function also suits the myth’s image of evils being contained and then released.
The familiar phrase arose partly through a Renaissance translation associated with Erasmus, who rendered the Greek container as a Latin word meaning box. That translation helped establish the wording used in English and many other languages. “Pandora’s jar” is therefore closer to Hesiod’s imagery, while “Pandora’s Box” is the established idiom.