I shared the halyard info because it is considered the most difficult. Sheets are easier as they are at ground level. There are a couple of thoughts.
For a main sheet some like to take a cheap piece of string and run it from the attachment point on the boom through all the blocks and to the control point in the cockpit. Then have your crew take hold of the boom swing it out all the way out over the dock till it just touches the shroud. Feed the string out while the boom is being swung. You now know the longest length of the Mainsheet. Add a few feet for tail in the cockpit and mark the line.
Now comes the fun part. Holding the end of the string in your left hand just under your chin and with your right hand stretch the dangling string out full reach. Measure the length of string that is in your hands. It should be about 3 feet and some inches. Whatever it is you now have a standard “stretch length”. Now you can simply pass the stretched string from your right hand to your left and repeat the stretch length measurement counting as you go how many times needed to get to the bitter end. When you get to the end of the string you know the needed length is X times the length of your first measured stretch. ( My stretch length is 3 feet times 30 stretches equals 90 feet of line is needed for the sheet. ). Extra benefit is you get to stretch and strengthen your arm muscles.
or
You can use the riggers approximate lengths found on a rope website like Premium ropes.
Rope length advice: these are our guidelines for the length of mooring lines, halyards and sheets for your boat.
www.premiumropes.com
You’ll get lines that are long enough or perhaps a bit too long and once on your boat may need some trimming. ( The trimmed bits can make excellent fender lines.) But you miss the first methods free rope exercise.
I use two criteria to identify the diameter of a sheet.
- The line must fit the sheave of the blocks the line passes through
- The line must feel good in hands
To small and you’ll hate the words “Trim the jib sheet “. Too big and it will jamb in the blocks. You’ll come to hate that as well. You want the Goldilocks sheets “Just the Right size”. Trouble is you try to ask the sales person at the rope store for Goldilocks sheets and they’ll give you a strange look. Let’s keep that just between us.