Mirrors are often used by organists. As the organ console in many cases isn't a very movable object.
The mirror at the church I play at is rather small. About the size of a car side view mirror. But since it sits quite close to me, I can see what I need to see.
Sizes of mirrors vary greatly, I've seen huge picture frame sized mirrors in large churches that hang above the organists head. Some churches also use a TV monitor.
Notre Dame de Paris is one example using a TV monitor. You can see glimpses of it here:
I vaguely recall seeing a vid where someone put a plasma TV above the organ console.