Hello
Welcome to Sailing! I have a San Juan 21 also. Below is what I use for lines, but there are some nicer high tech low stretch lines that are now available, that you might consider.
Main Sheet: 3/8” sta-set double braid (I think it is 60 ft)
Jib Sheets: 3/8” Sta-Set Double Braid, 40 ft
(secure middle of the sheet at the clew, so 20’ each side, same for genoa sheet)
130 Genoa Sheets: 3/8” Sta-Set Double Braid, 45 ft
Spinnaker Sheets: 1/4” 6mm double braid, two, 40 ft
(I bought one green for starboard, one red for port)
Main Halyard: 1/4” Sta-Set X (low stretch)
Jib Halyard: 1/4” Sta-Set X (low stretch)
Spinnaker Halyard: 1/4” Sta-Set X (low stretch)
Spinnaker Topping Lift 3/16” double braid
Spinnaker foreguy 1/4” double braid
Boom Topping Lift 3/16” double braid
(I don’t have the lengths handy, mine are longer than standard lines. I’ve run all of my lines to the cockpit, via turning blocks, to cam cleats)
Docking LInes:
Bow: 1/2” nylon 3-strand, 20 ft
Stern: 3/8” nylon 3-strand, 15 ft
My bow cleats (I added them) are larger than my stern cleats (stock).
If you need any San Juan 21 specific parts (like keel pivot, spreader, mast, gooseneck, etc) that you can’t find at marine retailers, here is a link for San Juan parts:
http://sjsailboats.com
Hope you get your boat sailing soon, . . well, maybe get it ready for next spring,
Jeff D.