Jib sheets.... 1.5 times boat length. OR the longest distance will run from the clew of the sail, all the way forward around the mast and shrouds and back to the winch on the other side, plus a few more feet. Most sailors just figure 1,5 x boat length. For a larger headsail, 150% genoa for example, or for an asymetrical spinnaker. 2 times boat length. It helps if you figure this out your self... visualize how the sheet will be used and where in relation to sail's position.
As far as diameter, 5/16 (8mm) minimum for your size boat. But for jib sheets I'd consider going one size larger, 3/8 (10 mm) because it's more comfortable in the hand. The winches and cleat will be a consideration.... for halyards, mainsheet and traveler, on your size boat, definitely 5/16. The smaller size will still fit the winch and cleats, the "hand" isn't as important as headsail sheets, but mostly the smaller diameter line will rund through the blocks at deck and mast a lot more smoothly, less friction, less work.
Line type for sheets, basic club level double braid polyester (dacron) such as New England Sta-set (not X), etc.. but I particularly like the Samson Trophy Braid which has a spun.... as in Fuzzy!.... cover. It is very comforable to hold and it's spun surface helps grip the winch better.
