Snellen Eye chart
This is the same chart that your doctor uses to determine your visual acuity.
The normal height for the letter A is 88 mm, and the viewing distance is 20 feet or 6 meters.
Download your free copy below.
The Snellen eye chart is usually read while standing at a distance of 20 feet or 6 meters. Acuity is represented as a fraction, with the distance at which you are standing being the top part of fraction (the numerator), and the normal maximum legible viewing distance as the denominator (bottom of fraction).
So if, at 20 feet, you can read the letters on the row marked "40", this means you have visual acuity of 20/40 or better: 1/2 normal. From 10 feet, if the smallest letters you could read were on the "40" line, this would give you an acuity of 10/40: 1/4 normal.
If you are nearsighted, your vision will become more normal the closer you stand to the chart.