I only hang small mirrors, up to about 4 square feet (such as 2′ x 2′, or, if round, about 2′ in diameter). If the mirror is larger than that, I must use an assistant, which requires special scheduling.
In no case will we hang a mirror heavier than about 50 lbs. While I realize it is not always possible for you to tell me the weight of the mirror in advance, please assume that if the mirror is over about 12 square feet (such as 3′ x 4′, or 2′ x 6′), we will not be able to hang it.
The reason for this is that, inch for inch, mirrors are usually about three times heavier than most framed paintings, prints, or photos. So if an average 3 ft. by 4 ft. painting weighs 25 lbs., a mirror of the same size is likely to weigh about 75 lbs.
And because it is not uncommon for a mirror to have been damaged during shipping, if the mirror is packed in a crate or other protective material please expect me to ask you to remain present while I unpack it.