Topic: Sheets & halyards
Yes, measurement would be easy, I would attach an endless (so you don't run out!) messenger line to end of halyard and then pull it through, then measure, and then you can pull back in if you aren't replacing right away, or then just pull the new one in. Take pictures or make notes as it's easy to forget what went where and why! ;-)
I also will soon replace my halyards and reef lines, and will likely just use a good quality line, not spectra/kevlar or wire or anything like that, I like high end dacron, and would just keep my old ones as emergency spares, as they aren't that bad, just the odd fray here and there.
As far as specs, I don't believe you can go up to 5/8 size very easily, due to sizes of clutches/cleats, etc....our lines are 3/8"?? gee, I see there are millimeters now, so I would just take a piece of mine to the store, and bring a piece of larger size to boat to see if anything fatter would fit on my self-tailing winches and through blocks, etc...before I bought anything. I would just research what the best one was and go with that (Dacron only, though).
The Contessa 26 "Seeadler" under "Sails for the Atlantic" in the forum replaced all his rigging with next size up, not sure if he did sheets as well, you could write him, and I will ask him too, as he has some great Contessa info/experiences!