You have to watch the model ship folks. "Rope", in the good old days of fighting sail was typically measured in circumference, not diameter.
Other than that, just get an idea of what size (diameter) you want in real world -- say 1" which is 64/64", so 1/35 of 64/64 gives you a bit less than 2/64" diameter which is about .03". Look them up in the ships catalog, or even better go to your local Michaels.