Take a tape measure and go from limb tip to limb tip around the archer side of the curve on the limbs with a straight line between limbs, then subtract 3". For a 68" bow it would be a 65" string usually.
For the hunting bows it might be 2.875" or so. Make sure you know if the bow can take anything but Dacron for a string. Most modern bows have fiberglass reinforcements at the tips to allow you to use fast flight or stiffer strings. Most of the older vintage bows require Dacron and/or Flemish twist strings to protect the limb tips.
This should get you in the ball park for any recurve. AMO 60 is the poundage at 28" draw length I'd suspect and not relevant other than you better start training seriously if you want to pull that back to 28+" with any sort of confidence. A 60lbs recurve hunting bow with a 60" or less length is a bear paw to draw unless you are super strong and fit at shooting recurves.
Cheers,
Pete