"è" is more common, but "sta" can also be used. It's just more common to use essere to refer to location.
Essere=to be; to exist. Stare=to stay and sometimes to remain. With these verbs, you often have to go beyond the literal translations and think idiomatically since both verbs have numerous usages. Both verbs can be used to express a location. In this translation, I only can infer DL is offering up stare to teach a new verb and its specific use for a permanent location as opposed to using essere for a more temporary location. Perhaps the translation could have provided a hint to the mirror's permanent status by referring to it being large and not an item easily relocated.