How does the mirror affect the eye chart in a visual acuity test?

No: The image is the same as if you were looking the entire 20 feet (or 6 meters). The virtual image produced by the mirror is 20 feet away. Now, if the mirror is very dirty or dusty, maybe the image would be slightly affected.

Lengthens light path: Most offices do not have a 20 foot lane. But standards and available charts are made to reflect a 20 foot lane. The way to handle this is to use mirror (s) to reflect the image the proper distance to approximate a 20 foot lane. The letter images can also be enlarged or diminished as needed but this will change the standardization. A 20 foot light path is the standard.