Physical Characteristics of Coins

It is important to recognize that a scanned image posted on the web does not give any information about the physical characteristics of the coin. Indications such as "200% of real size" or whatever I often see is not a useful measure since all of us don't use the same size of computer screen. If the dpi (dots per inch) of scanning is given then from the size of the image the diameter can be worked out. However it is best to give a diameter/thickness in mm and if available the weight in grams to to best available precision.