There's no foolproof method, but the older bows usually went by AMO bow length. Use a cloth tape measure and measure from the top of the string groove, along the contour of the limb, through the center of the riser, to the top of the other string groove--that will give you the AMO bow length (the same goes for a recurve). Your longbow should get a string about 3" shorter than the bow length. Since it's an older bow,only use a dacron string--flemish twist or endless will worth, but the material should be B-50 or B-500.

Chad