Roger,
I am sure there is a guide to this in the user manual showing lengths and suggested thicknesses. This has caught us out in the past, though, as we replaced both halyards with replacements of the recommended thickness only to find that a previous owner had used thicker ropes(easier on the hands)and fitted clam cleats to match, which did not cope with the thinner ropes under tension. Accordingly you may need to measure the actual thickness used on your boat.
If you do not have a copy of the manual let me know and I will look mine out and email you the relevant details.
David