5/16" is large enough for strength but too small for comfort for me. 3/8" is about as small as works for my hands but prefer 7/16". !/2" is nice but bulky and totally over strength for a boat your size. Purpose of the sail is another determinate. If it's a light air sail, you want the lightest sheets that will work. Regular use sails
, it's how comfortable you are using the line.
1.5 times boat length for spinnaker
and Genoa Halyards is a good place to start. Working/non overlapping jibs can get by with shorter line like the boat's length or even shorter.
I use dedicated lines for each jib. Buy line double the length needed for an individual sheet and run it through the sails clew equal distance. Use a Brummell splice to secure the line. This is the Brummell splice where you run each free end through the other side of line not the locking splice that is needed for the slippery high tech. lines. Using the Brummell splice makes tacking overlapping headsails a piece of cake as the sheets don't hang up on the shrouds like individual bowline secured sheets tend to do. You can also use a cow hitch to attach the line but that is a bit lumpy and snags a little as it passes over the shrouds though not nearly as much as bowlines.