Generally the eyeball is fine. I cannot imagine that Heller checked the scale.
The thread will be numbered. If you can find a table that makes sense to you, good luck. I never have, and there also are different systems in different countries.
A favorite thread unit of mine is the "denier". That's the number of grams that 9 kilometers weigh. About all I get from that is the higher the number, the thicker.
A couple of things. Black for standing, tan for running.
An apparent range of sizes has a nice effect. Standing rigging being the thickest.
The shrouds on the lower masts might be 6" diameter on the ship, which would be 0.75 mm or 1/32 inch.
That's not thread; that's cord.
The anchor cables might even be twice that size.
Try to find cotton.
Run it through beeswax.
Bill