the ratio is from the old 'film' days, 1:1 being life sized, ie 1" on the film = 1" in the subject ... 1:6 would be 1" on film being 6" in subject
so for digital, i guess it would vary depending on the actual size of the sensor in your camera, if its 2/3", then the width of the subject would be 2/3" to be 1:1 (for example)