This is just a random tidbit, but I'm a big fan of 19th century novels and the whole time period and I picked this up in a nonfiction book about Victorian houses.
A middle-class Victorian home *always* had a mirror over the mantle. There were no other options. It would cover the entire space - from the mantle to the ceiling, and from one side of the mantle to the other.
The key is that it "had" to be taller than it was wide. A mirror that was wider than it was tall was "common." I think the idea is that a "good" home had ceilings high enough that the mirror would be taller than it was wide.
The things people have chosen to care about over the years!