determining rope sizes and load factors using sisal rope

good resource for figuring sisal rope diamters

http://www.engineeringtoolbox.com/sisal-rope-strength-d_1517.html

also need to consider

http://www.engineeringtoolbox.com/wire-rope-slings-d_1613.html

and then there is the part about the users of the porch swing

you will not want to ask how much the users of the swing weigh usually
better to guesstimate