Ya need to make SURE that yer bow can safely handle modern low stretch,low creep string fibers. If ya don't know for SURE, stick with Dacron.
Then check with the bow maker for the recommended brace height and what AMO string length will best achieve that brace height.
Saxon Archery As a rough rule of thumb, a 60" r/d longbow probably could handle a 6-1/2" to 7-1/2" brace height range. If ya have a tillering string, it'll be easiest to set the string length - if not, start with a 57" string.
String center serving diameter size depends on the string fiber diameter, number of string strands, and yer arra nock throat size. In the best of all worlds, ya want a loose fit of the nock throat to the string's center serving - NEVER a tight fit.