In the video above, Peter mentions seeing a 2"x3" glass mirror flash at 26 mile range, which I can believe - the 1800's surveyor rule of thumb for a naked eye visible mirror flash was 10 miles per inch of side length for a square mirror, which would be roughly 24 miles for a 2"x3" mirror.

Similar results were reported by the U.S. Navy in 1951 signaling from the ground with a 3"x5" glass mirror in "good visibility" to a plane at 5,000 feet altitude reported ranges of:

45 miles with "the plane between the sun and the mirror"
30 miles with the sun at right angles
21 miles "down sun"

You can read about the 1951 Navy experiment in the article: "Mirror Flash Seen 45 Miles", Naval Aviation News, August 1951 page 28 {which is page 30 of the .pdf file), available here:
http://www.history.navy.mil/nan/backissues/1950s/1951/aug51.pdf

Here are videos I made of the mirror flashes from my 3"x5" glass mirror at ranges of 0.7, 11.1 and 43 miles: http://www.youtube.com/watch?v=GwCbgQGmID4

Please note such ranges generally require the air to be quite clear and/or thin - 43 miles is pushing it for a 3"x5" mirror in city atmosphere. I've run tests in more typical "light smog" weather when, despite moving one end to 5,689 ft altitude, and using a 12"x12" mirror, the flash was barely visible with binoculars at 48 mile range, and a 12"x12" glass mirror isn't the most practical piece of survival kit.

In my 43 mile test, both ends were at about 500' elevation, the air was very clear, and the sun to signal line range was 90 degrees. We could see each other's 3"x5" mirror flashes with the naked eye.