We tested signal mirrors today at 22 mile range, under favorable conditions, using:

(1) 3"x5" glass mirror ( GI issue MIL-M-18371E)
(2) 2"x3" plastic mirror ( American Medical Kits "Rescue Flash" model)
(3) 2"x3" stainless steel with aiming hole

I recorded each on video and put the first two on YouTube.

The 3"x5" (~15 sq. in) mirror was clearly visible at 22 miles with my naked eye - I called out dozens of flashes.

The 2"x3" (~6 sq. in) plastic mirror I saw about four times in 2 minutes with my naked eye. It wasn't because there weren't many times that many flashes heading my direction - you can see the flashes clearly in the video when I'm saying I can't see it on the sound track. I think it is just that with this smaller, less reflective mirror, only the strongest flashes were visible to me.

The 2"x3" (~6 sq. in) stainless mirror I didn't see at all with my naked eye - presumably because the metal was even less reflective than the plastic. it wasn't because it wasn't flashing in my direction - the flashes are clearly visible in the video when I'm saying I can't see it on the sound track. I also could see them when I used my 7x50 binoculars. I'm sure I'd have seen it easily at, say, ten mile range.

The air was quite clear - I could see mountains 94 miles away. The background was good and dark - you can see that in the video. He also had the advantage to signaling almost directly into the sun. Both ends were at about 1000 ft elevation.

I'd expect a bit more range if we were at higher altitude, or with stronger sun, but on the other hand I'd expect to lose range if we were at an angle to the sun.

The 3"x5" glass mirror video is here:
Background and consumer advice on this type of mirror is here:

The 2"x3" plastic Rescue Flash mirror video is here: http://www.youtube.com/watch?v=4JtfpaMiUqQ
This is widely available, e.g. at REI or REI.com.
A good discussion of the development of this mirror is about 40% down this page:

A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)