Measure the bow from tip to tip and then subtract 4 inches. This will give you a string in the ballpark. The big question is what should be the brace height?
Brace Height is measured from the centre of the bow grip to the bowstring at 90 degrees. Most bows will have a recommended brace height given by the manufacturer. If the brace height is not known, then a rough guide for recurves is:
70 inch bow = 8.50 to 9.75 inches,
68 inch bow = 8.25 to 9.50 inches,
66 inch bow = 8.00 to 9.25 inches.
64 inch bow = 7.75 to 9.00 inches.
The ideal brace height for a bow is one that produces the quietest release with the least amount of vibration in the bow. To adjust your brace height all you have to do is add twists to the sting to raise brace height and subtract twists to lower brace height.
Hope this helps you out some, if not let me know and I'll try and give you some more information.
Derek