In the earliest Greek version of Pandora’s story, Pandora opened a jar. Hesiod uses the Greek word pithos, a large storage vessel commonly used for commodities such as grain, wine, or oil.
The familiar “box” is a later development in the story’s transmission. The Renaissance humanist Erasmus helped popularize the Latin expression pyxis Pandorae, or “Pandora’s box,” when translating and discussing the Greek myth. That wording shaped the standard expression in many modern languages.
A pithos was much larger and more practical than the small hinged box shown in many illustrations. Artists and storytellers nevertheless continued to use boxes because they fit the later phrase and make the act of opening easier to depict.
Thus, “Pandora’s box” is the established idiom, but “jar” is the more accurate answer for Hesiod’s earliest surviving account.