When at sea off the southern California coast, I have often seen flashes from the windshields of vessels hull down on the horizon, otherwise not noticeable....

The largest "signal mirror" I have ever seen in use was a mirror from a standard bathroom cabinet, employed by gentlemen from Arizona Mine Safety to light up vertical shafts so that we could see clearly what was below, in many cases saving a tricky and somewhat dangerous vertical operation. I have used smaller mirrors as impromptu lights to illuminate dark corners in rock shelters and the mouths of caves- bigger is definitely better.