Measuring a chain depends on whether it is a structural chain, a chainsaw chain or a bike chain. A structural chain has to have its individual links measured (for strength) as well as the overall length. A chainsaw length must have its inner rollers measured to see what gearing it fits. A motorcycle chain must have all of the previous measurements made as well as width, because the chain return on bikes have close tolerances.
Measure a structural chain (for chandeliers or other decorative items) by taking a tape measure from one end of the chain to the other. If the chain is longer than your tape measure, you can find the length with individual link measures. Measure from one end of an individual link to the other end of that same link. Now count the links in the chain, and multiply that half of the chain length; this is because the links overlap, so this shortens the overall possible length. Also measure the width of the link's individual side to find its diameter. This can be used to determine the tensile strength of the chain if you know the material it is made of.
Measure a chainsaw chain by measuring the length of the chain with a measuring tape, then count the drive links in the chain; the drive links are links with gear teeth (as opposed to the cutting links that have the blades). Chainsaw chains are measured by the number of drive teeth in most circumstances but also in length. Now measure the blade height on the individual blade links to determine the blade size.
Measure a motorcycle chain by first measuring the overall length, hen measure the individual links by measuring the distance between the roller. With the roller distance, you can find the gearing ratio for your model of bike (you will need to consult your bike's manual). Now measure the width of the chain at its thickest point (where the rollers are connected by an out link, not an inner linkage). Some motorcycles, or those with modified chain returns, can only take smaller widths.