A standard bow string length depends on the length of the bow. For example, a 60-inch bowstring is for a 60-inch bow. The bowstring itself is not 60 inches in length, but has been measured according to the Archery Trade Association's specifications for use with a 60-inch bow.
The Archery Trade Association's standard for measuring bowstrings for bows over 40 pounds includes placing the string over the steel pegs and stretching it at 100 pounds of tension for a time period of 20 seconds. Take the string measurement from the outside of the pegs. For bows under 40 pounds, use the same technique but modify the amount of tension applied to 50 pounds.